MINOR: h2: keep a count of the number of conn_streams attached to the mux

MINOR: h2: keep a count of the number of conn_streams attached to the mux

The h2 mux only knows about the number of H2 streams which are not in a
CLOSED state. This is used for protocol compliance. But it doesn't hold
the number of really attached streams. It is a problem because depending
on scheduling, it is possible that more streams are attached to the mux
than the ones seen at the protocol level, due to some streams taking some
time to be detached. Let's add this count based on the conn_streams.

Note: this patch is part of a series of fixes which will have to be
backported to 1.8.