BUG/MEDIUM: mux-h2: Restart reading when mbuf ring is no longer full

BUG/MEDIUM: mux-h2: Restart reading when mbuf ring is no longer full

When the mbuf ring buffer is full, the flag H2_CF_DEM_MROOM is set on the H2
connection to block any demux. It is important to properly handle ACK
frames. However, we must take care to restart reading when some data were
removed from the mbuf. Otherwise, we may block the demux for no reason. It
is especially an issue if the demux buffer is full. In that case, the H2
connection is blocked, waiting for the timeout.

This patch should be backported to 3.2. But is is probably a good idea to
not backport it on older versions, except if a bug is reported in this area.