osd/PG: support is_locked() in non-debug mode

osd/PG: support is_locked() in non-debug mode

`PG` print details info in the prefix of logging messages if the PG is
being locked by current thread. but `ceph::mutex` is an alias of
`std::mutex` in non-debug mode, so neither `mutex::is_locked_by_me()` nor
`mutex::is_locked()` is supported in non-debug mode. to continue supporting
this feature, `PG::locked_by` is added to memorize the thread id of the owner
of the lock.

Signed-off-by: Kefu Chai <kchai@redhat.com>