bitbake: runqueue: Fix event timing race

bitbake: runqueue: Fix event timing race

The event from the task notifiing of hash equivalency should only be processed
when the task completes. This can otherwise result in a race where a dependent
task may run before the original task completes causing various failures.

To make this work reliably, the code had to be restructured quite a bit.

(Bitbake rev: 1bf5be46f92f125193638cf41ff207d68f592259)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>