Support nested locks for the same lock on the same connection

[rsyring]

PG advisory locks support nested locks. So, the same connection can request the same lock multiple times. We don't currently support that, but should consider it.

[jobh]

Even if not supported, the current behaviour is inconsistent - the lock can be acquired multiple times, but only released once:

>>> locker = pals.Locker('test', url)                                                                                                                   
>>> lock1, lock2 = locker.lock('t1'), locker.lock('t1')
>>> lock1.acquire(blocking=False)
True
>>> lock1.acquire(blocking=False)
True
>>> lock1.release()
True
>>> lock1.release()
False
>>> lock2.acquire(blocking=False)
False
>>> del lock1
>>> lock2.acquire(blocking=False)
True