Cap total number of concurrent requests per HTTP/2 connection

[arturobernalg]

This change adds an optional hard cap on the total number of queued / in-flight requests per HTTP/2 connection. When the per-connection limit is reached, new submissions fail fast with RejectedExecutionException and the slot is released when the exchange handler releases its resources. The cap is configured via H2MultiplexingRequesterBootstrap and passed into H2MultiplexingRequester at construction time to keep the requester configuration immutable and avoid API incompatibilities.

[ok2c]

@arturobernalg I do not like this approach. There is already a command queue built into the IOSession. We should leverage that queue instead of creating another layer of complexity. What we need is to extend IOSession interface to expose information about already pending commands and optionally reject commands if the queue is over a particular limit.