replace in_method_t with a counter

A single counter ( waiting ) accurately reflects the number of
waiters, regardless of the method waiting. It is enough to allow
unit tests to synthetise all situations, including:

T1: x = lookup_or_create(0)
T1: release x part 1 (weak_ptrs now fail to lock)
T2: y = lookup_or_create(0)
T2: block in lookup_or_create (waiting == 1)
T1: z = lookup_or_create(1) (does not block because the key is different)
    while holding the lock it waiting++ and waiting == 2
    and before returning it waiting-- and waiting is back to == 1
T1: complete release x
T2: complete lookup_or_create(0) (waiting == 0)

The unit tests are modified to add a lookup on an unrelated key to
demonstrate that it does not reset waiting counter.

http://tracker.ceph.com/issues/5527 refs #5527

Signed-off-by: Loic Dachary <loic@dachary.org>