MINOR: mux: Add a new "avail_streams" method.

Add a new method for mux, avail_streams, that returns the number of streams
still available for a mux.
For the mux_pt, it'll return 1 if the connection is in idle, or 0. For
the H2 mux, it'll return the max number of streams allowed, minus the number
of streams currently in use.
3 files changed