BUG/MINOR: mux_quic: limit avail_streams() to 2^62

BUG/MINOR: mux_quic: limit avail_streams() to 2^62

QUIC streams ID are encoded as 62-bit integer and cannot reuse an ID
within a connection. This is necessary to take into account this
limitation for backend connections.

This patch implements this via qmux_avail_streams() callback. In the
case where the connection is approaching the encoding limit, reduce the
advertised value until the limit is reached. Note that this is very
unlikely to happen as the value is pretty high.

This should be backported up to 3.3.