MINOR: queue: centralize dequeuing code a bit better

MINOR: queue: centralize dequeuing code a bit better

For now the pendconns may be dequeued at two places :
  - pendconn_unlink(), which operates on a locked queue
  - pendconn_free(), which operates on an unlocked queue and frees
    everything.

Some changes are coming to the queue and we'll need to be able to be a
bit stricter regarding the places where we dequeue to keep the accounting
accurate. This first step renames the locked function __pendconn_unlink()
as it's for use by those aware of it, and introduces a new general purpose
pendconn_unlink() function which automatically grabs the necessary locks
before calling the former, and pendconn_cond_unlink() which additionally
checks the pointer and the presence in the queue.