MINOR: queue: make sure the pendconn is released before logging

MINOR: queue: make sure the pendconn is released before logging

We'll soon need to rely on the pendconn position at the time of dequeuing
to figure the position a stream took in the queue. Usually it's not a
problem since pendconn_free() is called once the connection starts, but
it will make a difference for failed dequeues (eg: queue timeout reached).
Thus it's important to call pendconn_free() before logging in cases we are
not certain whether it was already performed, and to call pendconn_unlink()
after we know the pendconn will not be used so that we collect the queue
state as accurately as possible. As a benefit it will also make the
server's and backend's queues count more accurate in these cases.