BUG/MINOR: queue/threads: make the queue unlinking atomic

BUG/MINOR: queue/threads: make the queue unlinking atomic

There is a very short race in the queues which happens in the following
situation:
  - stream A on thread 1 is being processed by a server
  - stream B on thread 2 waits in the backend queue for a server
  - stream B on thread 2 is fed up with waiting and expires, calls
    stream_free() which calls pendconn_free(), which sees the
    stream attached
  - at the exact same instant, stream A finishes on thread 1, sees
    one stream is waiting (B), detaches it and wakes it up
  - stream B continues pendconn_free() and calls pendconn_unlink()
  - pendconn_unlink() now detaches the node again and performs a
    second deletion (harmless since idempotent), and decrements
    srv/px->nbpend again

=> the number of connections on the proxy or server may reach -1 if/when
   this race occurs.

It is extremely tight as it can only occur during the test on p->leaf_p
though it has been witnessed at least once. The solution consists in
testing leaf_p again once the lock is held to make sure the element was
not removed in the mean time.

This should be backported to 2.0 and 1.9, probably even 1.8.