swarming: Despaghettify bot_config hook calls.

[vadimsht]

Bot startup process is messy.

I'll be taking specifically about Chrome infra bot_config.py case, as a primary example.

Here's how it works:

• bot_config.get_dimensions and bot_config.get_state are called with bot.Bot object that has id == 'none' (and the rest of the properties missing). bot_config.get_dimensions is actually using android.get_devices, but Android subsystem hasn't been initialized yet (because it is initialized in on_bot_startup), and so initial dimensions and state are bogus.
• Then on_bot_startup is called. It assumes bot.Bot object already has dimensions set (which are set to possibly bogus values, since Android subsystem hasn't been initialized yet). It initializes Android subsystem.
• bot_config.get_dimensions and bot_config.get_state are called again. Now with correct bot.Bot object and correct Android subsystem state.
• Finally /handshake endpoint is called, it receives correct dimensions and state.

This is at least very confusing (bordering on "easy to break the world", e.g. https://chromereviews.googleplex.com/465757013).

I'm now trying to return additional data from /handshake, that can be used in get_dimensions to make decisions. Unfortunately, get_dimensions is called twice in different circumstances even before /handshake is called. Adding third flavor of environment in which get_dimensions is called is too much.

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Proposal:

1. Postulate that on_bot_startup is called before any other hook (including before get_dimensions).
2. Document that on_bot_startup receives special uninitialized bot.Bot object.
3. Teach on_bot_startup to return bot ID, initial set of dimensions and state, send them to the /handshake.
4. Document that /handshake receives initial dimensions and state that may be changed in the future by get_dimensions and get_state.
5. Document that get_dimensions and get_state are called immediately before /poll (e.g. after all initialization and initial handshake).
6. Forbid ID dimension to be changed by get_dimensions (bot changing its ID midlife doesn't make much sense).

The new flow then would be:

• on_bot_startup is called (with bot.Bot.id == 'none'), it initializes Android subsystem and bot ID and initial dimensions and state.
• /handshake is called. It may return additional server-supplied data, fetched based on bot's ID.
• get_dimensions and get_state are called. They may use state set up in on_bot_startup and data returned by /handshake.
• /poll

[maruel]

About 2., maybe not pass a Bot object?

lgtm, thanks for looking at this.