Bloc doesn't await for async side effects completion before closing

[pierremrtn]

Issue raised from Tango-Tango fork:

Since StateMachine handles onEnter/onExit/onChange methods
in the [Bloc.onChange] handler, these side effects cannot be awaited.

Due to this, the bloc has no understanding of when these effects may be
completed, and they aren't treated like normal event handlers
that are run to completion before a bloc is closed.