osdc/Objecter: use SafeTimer; make callbacks race-tolerant

The RWTimer event cancellation is racy.  Instead, just make all of our
callbacks tolerate cancellation races.  This is already true of most of
them (in fact, they are probably broken because they try to take a write
lock while holding a read lock).  Fix C_CancelOp so that it calls the
other op_cancel (that takes a tid).

Then switch the RWTimer back to a SafeTimer.  Put it in unsafe callbacks
mode because we don't want to introduce lock cycles with timer_lock.

Fixes: #9582
See also: #9650
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2631 from ceph/wip-rwtimer

DNM: osdc/Objecter: use SafeTimer; make callbacks race-tolerant

Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>