Flow control needs improvements.
Merge branch 'jetty-http2' into http2_flow_control
Fixed handling of the close state in case it's already closed.
Refactored onReadTimeout() to parent class.
Reworked flow control implementation. Splitted HTTP2Flusher out of HTTP2Session.

Flow control window updates are now processed by the flusher, so that
it is the only component that handles window updates.
In the process of this refactoring, HTTP2Flusher was refactored out
of HTTP2Session.
Merged branch 'jetty-http2' into 'http2_flow_control'