Allow adjustment of per-connection concurrent stream limits

[Ralith]

Fixes #1313.

This is useful when multiple application protocols are hosted on the same endpoint, or when application logic such as authentication might want to adjust the limits of pre-existing connections.

[Ralith]

On review, the previous implementation had significant bugs:

• ensure_remote_streams was not idempotent
• MAX_STREAMS frames sent in 0-RTT would not be retransmitted if 0-RTT was rejected

I've adjusted the implementation to resolve those by:

• Counting allocated remote streams rather than concurrently opened ones
• Testing that we allocate the expected number of streams on set_max_concurrent calls
• Explicitly tracking whether flow control was adjusted during 0-RTT

Please re-review.

[sakridge]

Any updates on this?

[djc]

Sorry, I should review this. Will do that today (I'm on Pacific time right now).