Consolidate Handler/PendingHandler

[khionu]

Current concerns:

• Panic handling is currently not done during poll
• The flow, from start to finish of a message processing, is currently distributed in a way that makes it hard to keep changes in sync
• As an async operation, each iteration of the ActorStream can and should have everything related to the pending outcome wrapped in a single Future.

[khionu]

After further reflection, there really isn't a lot that can be done about this. We can make sure the Handler has all the panic handling inside it, but other than that, consolidation would create more problems than it would solve.