We need a ’Load’ lifecycle

[swankjesse]

In our use of Zipline we’re doing fancy things:

1. As soon as the ZiplineManifest is ready, we inspect the metadata for URLs and download them. These downloads occur in parallel with code downloads.
2. When the rest of the code is ready we bind a ZiplineService that exposes these downloads to guest code.

Unfortunately the implementation of this is clumsy. We’re hooking into EventListener, using manifestReady() for 1 and ziplineCreated() for 2. This isn’t what EventListener is for!

I’d prefer a proper LoadAttempt class with support for a user-specified tag.