MEDIUM: queue: use a trylock on the server's queue

MEDIUM: queue: use a trylock on the server's queue

Doing so makes sure that threads attempting to wake up new connections
for a server will give up early if another thread is already in charge
of this. The goal is to avoid unneeded contention on low server counts.

Now with a single server with 16 threads in roundrobin we get the same
performance as with multiple servers, i.e. ~575kreq/s instead of ~496k
before. Leastconn is seeing a similar jump, from ~460 to ~560k (the
difference being the calls to fwlc_srv_reposition).

The overhead of process_srv_queue() is now around 2% instead of ~20%
previously.