MINOR: queue: make pendconn_first() take the lock by itself

MINOR: queue: make pendconn_first() take the lock by itself

Dealing with the queue lock in the caller remains complicated. Let's
change pendconn_first() to take the queue instead of the tree head,
and handle the lock itself. It now returns an element with a locked
queue or no element with an unlocked queue. It can avoid locking if
the queue is already empty.