This implements stream data handling using a buffer between the H/2
session thread and each stream thread. This is needed to avoid head of
line blocking on the session socket when a data frame is received for a
stream thread that is not yet ready to receive it.

The buffer used will have to be as large as the send window the peer
expects at the time the stream is opened. This will typically be 65535
unless the h2_initial_window_size parameter has been changed.

Stream window updates will then be issued only once data is removed from
the buffer by the request body being consumed from the request handling
thread, limited in size to what space is then available in the buffer.

Conflicts:
	bin/varnishd/http2/cache_http2_proto.c