HTTP/2 client-side meta

[jrudolph]

This is a summary ticket for all things missing for public HTTP/2 client support after #3166 has been merged.

• HTTP/2 client: more complete test suite #3227 - more tests for more complex interactions (multiplexed request/response streams, error cases)
• HTTP/2 client: backpressure requests when server (peer) announced max-concurrent-streams is reached / Respect peer SETTINGS_MAX_CONCURRENT_STREAMS #3228 - backpressure new user requests when server announced max-concurrent-streams are reached
• HTTP/2 client: review rendering parsing #3229 - review whether parsing and rendering need more complete implementations, DRY with server side
• HTTP/2 client: public connection-level API #3230 - public connection-level user API
• HTTP/2 client: integrate into shared pool API #3231 - integration with pool API
• HTTP/2 demux: unify incoming and outgoing stream handling #3232 - revamp stream handling in demux: for less confusion, incoming and outgoing stream implementations should be merged as far as possible
• HTTP/2: investigate potential stream leak #3233 - investigate potential stream leak (might be more likely on server side?)
• Load balancing pool for HTTP/1.1 and HTTP/2 #3505 - load-balancing API