refactor(bloc_concurrency): `exhaustMap` from stream_transform

[purplenoodlesoop]

Hello, folks!

I've opened an issue in the stream_transform package about adding the exhaustMap Stream extension.

The droppable concurrency mode currently uses a custom implementation of this transformer, and since my issue in the stream_transform packages seems to be taken into work, when it is implemented, I think the droppable mode should be refactored to use not the homebrewed version, but the version from the stream_transform.