BUG/MEDIUM: mux-h2/htx: Always set CS flags before exiting h2_rcv_buf()

BUG/MEDIUM: mux-h2/htx: Always set CS flags before exiting h2_rcv_buf()

It is especially important when some data are blocked in the RX buf and the
channel buffer is already full. In such case, instead of exiting the function
directly, we need to set right flags on the conn_stream. CS_FL_RCV_MORE and
CS_FL_WANT_ROOM must be set, otherwise, the stream-interface will subscribe to
receive events, thinking it is not blocked.

This bug leads to connection freeze when everything was received with some data
blocked in the RX buf and a channel full.

This patch must be backported to 1.9.