Client: make sure the Finisher's mutex lock not held during it being distructed

Client: make sure the Finisher's mutex lock not held during it being distructed

The objecter_finisher is already started in Client::Client(), but
in the failure path when initializing and starting the Client object,
we may not get a chance to call the Client::shutdown() to stop the
Finisher thread, which maybe still holding the mutex lock in it. Then
when destrucing the Finisher object the pthread_mutex_destroy() will
fail.

This fix will delay the objecter_finisher thread to start in ::init()
until we're ready to call Client::shutdown on any errors instead.

Fixes: https://tracker.ceph.com/issues/44389
Signed-off-by: Xiubo Li <xiubli@redhat.com>