HTTP/3: fixed ngx_stat_active counter.

HTTP/3: fixed ngx_stat_active counter.

Previously the counter was not incremented for HTTP/3 streams, but still
decremented in ngx_http_close_connection().  There are two solutions here, one
is to increment the counter for HTTP/3 streams, and the other one is not to
decrement the counter for HTTP/3 streams.  The latter solution looks
inconsistent with ngx_stat_reading/ngx_stat_writing, which are incremented on a
per-request basis.  The change adds ngx_stat_active increment for HTTP/3
request and push streams.