BUG: backend: stop looking for queued connections once there's no more

BUG: backend: stop looking for queued connections once there's no more

Commit ae0b12ee0 ("MEDIUM: queue: use a trylock on the server's queue")
introduced a hard to trigger bug that's more visible with a single thread:
if a server dequeues a connection and finds another free slot with no
connection to place there, process_srv_queue() will never break out of
the loop. In multi-thread it almost does not happen because other threads
bring new connections.

No backport is needed as it's only in -dev.