MINOR: xprt: add new xprt_set_idle and xprt_set_used methods

MINOR: xprt: add new xprt_set_idle and xprt_set_used methods

These functions are used on the mux layer to indicate that the connection
is becoming idle and that the xprt ought to be careful before checking the
context or that it's not idle anymore and that the context is safe. The
purpose is to allow a mux which is going to release a connection to tell
the xprt to be careful when touching it. At the moment, the xprt are
always careful and that's costly so we want to have the ability to relax
this a bit.

No xprt layer uses this yet.