BUG/MEDIUM: h2: fix stream limit enforcement

BUG/MEDIUM: h2: fix stream limit enforcement

Commit 4974561 ("BUG/MEDIUM: h2: enforce the per-connection stream limit")
implemented a stream limit enforcement on the connection but it was not
correctly done as it would count streams still known by the connection,
which includes the lingering ones that are already marked close. We need
to count only the non-closed ones, which this patch does. The effect is
that some streams are rejected a bit before the limit.

This fix needs to be backported to 1.8.