Adds the initial DefaultMessageManager implementation

[ashovlin]

Issue #, if available: DOTNET-6664

Description of changes: Adds the message manager, which was the last missing component for the subscriber workflow!

Some loose ends since I'm rushing to get this out before vacation:

• We're now able to write integration tests more easily, though none are included here. We might want to break into a separate task. I've done some manual testing of single message workflows.
• There's a TODO in the code, I'm unsure if we want to plumb the CancellationToken for the entire poller down into a single message's handling (and therefore its delete or visibility extension calls).
• The unit test that a 3 second message handling with a 1 second visibility window will call extend either 2 or 3 times. I used Between so it always passes for now, but might need to tighten it up.
• I haven't done any unit or manual/integration testing for more then one message in the queue at a time, so the synchronization around ActiveMessageCount hasn't been exercised much.

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[normj]

Just one small method rename but otherwise I'm good.