workers/gthread: Remove locks + one event queue + general cleanup #3157
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The main purpose is to remove complexity from gthread by:
Removing the lock for handling self._keep and self.poller. This is possible since we now do all such manipulation on the main thread instead. When a connection is done, it posts a callback through the PollableMethodCaller which gets executed on the main thread.
Having a single event queue (self.poller), as opposed to also managing a set of futures. This fixes Gunicorn gthread deadlock #3146 (although there are more minimal ways of doing it).
There are other more minor things as well:
Some complexity has been added to the shutdown sequence, but hopefully for good reason: it's to make sure that all already accepted connections are served within the grace period.