MEDIUM: queue: make pendconn_process_next_strm() only return the pendconn

MEDIUM: queue: make pendconn_process_next_strm() only return the pendconn

It used to do far too much under the lock, including waking up tasks,
updating counters and repositionning entries in the load balancing algo.

This patch first moves all that stuff out of the function into the only
caller (process_srv_queue()). The decision to update the LB algo is now
taken out of the lock. The wakeups could be performed outside of the
loop by using a local list.

This increases the performance from 377k to 431k req/s on 16 threads
with 20 servers under leastconn.

The perf profile changes from this:
  23.17%  haproxy             [.] process_srv_queue
   6.58%  haproxy             [.] pendconn_add
   6.40%  haproxy             [.] pendconn_dequeue
   5.48%  haproxy             [.] fwlc_srv_reposition
   3.70%  haproxy             [.] fwlc_get_next_server

to this:
  13.95%  haproxy             [.] process_srv_queue
   9.96%  haproxy             [.] fwlc_srv_reposition
   6.21%  haproxy             [.] fwlc_get_next_server
   3.96%  haproxy             [.] pendconn_dequeue
   3.75%  haproxy             [.] pendconn_add