Add an article about multiplexing (#465)

Motivation:

It's hard to keep track of all the pieces that do multiplexing. When
making some changes I found myself writing down what each piece did and
how they fit together. I think this is generally useful information for
anyone maintaining the library.

Modifications:

- Add a DocC article about the different multiplexing approaches that's
aimed at _maintainers_ of NIOHTTP2.

Result:

Easier to learn about how multiplexing is done

---------

Co-authored-by: Cory Benfield <[email protected]>

Sources/NIOHTTP2/Docs.docc/Articles/Multiplexing-for-maintainers.md

@@ -0,0 +1,98 @@
+# Multiplexing for maintainers
+
+The term "multiplexer" appears in many types across ``NIOHTTP2``. This article
+explains the different ways stream multiplexing is achieved and how the pieces
+fit together.
+
+The intended audience is _library maintainers_ although users may find it
+helpful too. It is not meant to be a comprehensive guide and assumes you have at
+least a high-level understanding of how the library is structured.
+
+There are two high-level approaches to multiplexing in ``NIOHTTP2``:
+
+1. _Legacy_ multiplexing is implemented as a separate `ChannelHandler`.
+2. _Inline_ multiplexing is implemented within the ``NIOHTTP2Handler``.
+
+## Legacy
+
+The _legacy_ multiplexer is a separate `ChannelHandler` which maintains the set
+of open child channels keyed by their stream ID. It is implemented as the
+``HTTP2StreamMultiplexer``. The handler receives frames and various channel
+events from the ``NIOHTTP2Handler`` which are acted upon or propagated to the
+appropriate child channel or forwarded down the connection channel. This
+approach makes heavy use of user-inbound events to pass additional information
+between the ``NIOHTTP2Handler`` and the ``HTTP2StreamMultiplexer``.
+
+The child channels are wrapped up as `MultiplexerAbstractChannel`s: this is a
+wrapper around an `HTTP2StreamChannel` operating in one of two possible modes.
+The modes correspond to the type of data passed between the connection channel
+and the stream channels:
+
+1. ``HTTP2Frame/FramePayload``
+2. ``HTTP2Frame``. All paths leading to this mode of operation are deprecated,
+   see [Issue 214](https://github.com/apple/swift-nio-http2/issues/214) for more
+   context.
+
+## Inline
+
+`_Inline_` multiplexing is the newer way of multiplexing streams and is
+implemented inline in the ``NIOHTTP2Handler``. It was created to reduce the cost
+of multiplexing incurred by the legacy multiplexer by removing the out-of-band
+passing of information as user-inbound events. Like the legacy multiplexer, it
+maintains a set of open stream channels and propagates events to them directly.
+This is implemented as the `NIOHTTP2Handler.InlineStreamMultiplexer`.
+
+## How they fit together
+
+Both methods are supported which goes some of the way to explaining why there
+are so many types with the word "multiplexer" in their name.
+
+The ``NIOHTTP2Handler`` makes it possible to use either approach by abstracting
+them away behind the `HTTP2StreamMultiplexer` protocol. The two implementations
+(`LegacyInboundStreamMultiplexer` and `InlineStreamMultiplexer`) are wrapped up
+in an `enum` called `HTTP2InboundStreamMultiplexer` held by the
+``NIOHTTP2Handler``.
+
+The `LegacyInboundStreamMultiplexer` just forwards events down the channel
+pipeline to the ``HTTP2StreamMultiplexer`` `ChannelHandler` which in turn calls
+into the `HTTP2CommonInboundStreamMultiplexer`.
+
+The `NIOHTTP2Handler.InboundStreamMultiplexer` calls the
+`HTTP2CommonInboundStreamMultiplexer` directly.
+
+The ``NIOHTTP2Handler`` propagates events to `HTTP2InboundStreamMultiplexer` but
+_also_ forwards events down the channel pipeline. Because of this, and to avoid
+events being delivered twice, some events on the
+`LegacyInboundStreamMultiplexer` are no-ops.
+
+## The relevant pieces, briefly(-ish)
+
+- **``NIOHTTP2Handler``**: a `public` `ChannelHandler` which decodes bytes to
+  ``HTTP2Frame``s (and vice versa). When operating in the 'inline' mode outbound
+  streams can be _created_ (via ``NIOHTTP2Handler/StreamMultiplexer``). Inbound
+  streams are demultiplexed via the `NIOHTTP2Handler.InboundStreamMultiplexer`.
+- **`HTTP2InboundStreamMultiplexer`**: an `internal` `protocol` which
+  demuiltiplexes various inbound events (receiving frames, errors, etc.) into
+  streams.
+- **`NIOHTTP2Handler.InboundStreamMultiplexer`**: an `internal` `enum` with two
+  cases conforming to `HTTP2InboundStreamMultiplexer`. The `NIOHTTP2Handler`
+  holds an instance of this internally.
+- **``NIOHTTP2Handler/StreamMultiplexer``**: a `public` `struct` which wraps
+  `HTTP2InboundStreamMultiplexer` to provide API for creating outbound streams.
+- **``HTTP2StreamMultiplexer``**: a `public` `ChannelHandler` used for _legacy_
+  multiplexing. which also allows you to _create_ outbound streams. Most of its
+  internals call through to the `HTTP2CommonInboundStreamMultiplexer`.
+- **`LegacyInboundStreamMultiplexer`**: an `internal` `struct` which conforms to
+  `HTTP2InboundStreamMultiplexer` and forwards various events down the channel
+  pipeline (to the `HTTP2StreamMultiplexer` handler). One of the two cases of
+  the `NIOHTTP2Handler.InboundStreamMultiplexer`.
+- **`HTTP2CommonInboundStreamMultiplexer`**: an `internal` `struct` which
+  actually holds the set of open streams keyed by their ID and is responsible
+  for propagating events to them.
+- **`InlineStreamMultiplexer`**: an `internal` `struct` which conforms to
+  `HTTP2InboundStreamMultiplexer` and forwards events directly to the
+  `HTTP2CommonInboundStreamMultiplexer`. The second case of the
+  `NIOHTTP2Handler.InboundStreamMultiplexer`.
+- **`MultiplexerAbstractChannel`**: an `internal` `enum` with two cases which
+  wraps an `HTTP2StreamChannel`. The two cases correspond to the type of data
+  passed into the stream (``HTTP2Frame`` vs. ``HTTP2Frame/FramePayload``).