HTTP/2: fixed reusing connections with active requests.

HTTP/2: fixed reusing connections with active requests.

New connections are marked reusable by ngx_http_init_connection() if there
are no data available for reading.  As a result, if SSL is not used,
ngx_http_v2_init() might be called when the connection is marked reusable.
If a HEADERS frame is immediately available for reading, this resulted
in connection being preserved in reusable state with an active request,
and possibly closed later as if during worker shutdown (that is, after
all active requests were finalized).

Fix is to explicitly mark connections non-reusable in ngx_http_v2_init()
instead of (incorrectly) assuming they are already non-reusable.

Found by Sergey Kandaurov.