BUG/MINOR: mux-h1: fix the open streams counter on stream creation failure

BUG/MINOR: mux-h1: fix the open streams counter on stream creation failure

The "h1_open_streams" stats counter is incremented at the end of
h1c_frt_stream_new() and h1c_bck_stream_new(), and decremented in
h1s_destroy(). But when the stream connector attachment or the sedesc
allocation fails after h1s_new() succeeded, h1s_destroy() is called on the
error path and decrements the counter which was not incremented. The gauge
then drifts and may become negative.

The counter is now incremented in h1s_new() as soon as the H1 stream is
allocated, so that it is always balanced by h1s_destroy(). The total streams
counter is left where it was so that it only counts successfully created
streams.

This patch may be backported to all supported versions.