osdc/Filer: use finisher to execute C_Probe and C_PurgeRange

Currently contexts C_Probe/C_PurgeRange are executed while holding
OSDSession::completion_lock. C_Probe and C_PurgeRange may call
Objecter::stat() and Objecter::remove() respectively, which acquire
Objecter::rwlock. This can cause deadlock because there is intermediate
dependency between Objecter::rwlock and OSDSession::completion_lock:

 Objecter::rwlock -> OSDSession::lock -> OSDSession::completion_lock

The fix is exexcute C_Probe/C_PurgeRange in finisher thread.

Fixes: #10229
Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #3074 from ceph/wip-10229

osdc/Filer: use finisher to execute C_Probe and C_PurgeRange