MINOR: h2: make each H2 stream support an intermediary input buffer

MINOR: h2: make each H2 stream support an intermediary input buffer

The purpose is to decode to a temporary buffer and then to copy this buffer
to the caller upon request to avoid having to process frames on the fly
when called from the higher level. For now the buffer is only initialized
on stream creation via cs_new() and allocated if the buffer_wait's callback
is called.