MEDIUM: queue: move the queue lock manipulation to pendconn_process_next_strm()

MEDIUM: queue: move the queue lock manipulation to pendconn_process_next_strm()

By placing the lock there, it becomes possible to lock the proxy
later and to unlock it earlier. The server unlocking also happens slightly
earlier.

The performance on roundrobin increases from 481k to 524k req/s on 16
threads. Leastconn shows about 513k req/s (the difference being the
take_conn() call).

The performance profile changes from this:
   9.32%  hap-pxok            [.] process_srv_queue
   7.56%  hap-pxok            [.] pendconn_dequeue
   6.90%  hap-pxok            [.] pendconn_add

to this:
   7.42%  haproxy             [.] process_srv_queue
   5.61%  haproxy             [.] pendconn_dequeue
   4.95%  haproxy             [.] pendconn_add