MINOR: muxes: Add ctl commands to get info on streams for a connection

MINOR: muxes: Add ctl commands to get info on streams for a connection

There are 2 new ctl commands that may be used to retrieve the current number
of streams openned for a connection and its limit (the maximum number of
streams a mux connection supports).

For the PT and H1 muxes, the limit is always 1 and the current number of
streams is 0 for idle connections, otherwise 1 is returned.

For the H2 and the FCGI muxes, info are already available in the mux
connection.

For the QUIC mux, the limit is also directly available. It is the maximum
initial sub-ID of bidirectional stream allowed for the connection. For the
current number of streams, it is the number of SC attached on the connection
and the number of not already attached streams present in the "opening_list"
list.