BUG/MINOR: mux-h1: Increment open_streams counter when H1 stream is created

BUG/MINOR: mux-h1: Increment open_streams counter when H1 stream is created

Since this counter was added, it was incremented at the wrong place for
client streams. It was incremented when the stream-connector (formely the
conn-stream) was created while it should be done when the H1 stream is
created. Thus, on parsing error, on H1>H2 upgrades or TCP>H1 upgrades, the
counter is not incremented. However, it is always decremented when the H1
stream is destroyed.

On bakcned side, there is no issue.

This patch must be backported to 2.6.