MINOR: mux-h2: add rxbuf head/tail/count management for h2s

MINOR: mux-h2: add rxbuf head/tail/count management for h2s

Now the h2s get their rx_head, rx_tail and rx_count associated with the
shared rxbufs. A few functions are provided to manipulate all this,
essentially allocate/release a buffer for the stream, return a buffer
pointer to the head/tail, counting allocated buffers for the stream
and reporting if a stream may still allocate.

For now this code is not used.