client: use finisher to abort MDS request

client: use finisher to abort MDS request

When a request is interrupted, libfuse first locks an internal mutex,
then calls the interrupt callback. libfuse need to lock the same mutex
when unregistering interrupt callback. We unregister interrupt callback
while client_lock is locked, so we can't acquiring the client_lock in
the interrupt callback.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
(cherry picked from commit 09699454e729592d426aeff5b578697e850af12e)

Conflicts:
	src/client/Client.cc
	src/client/Client.h

Signed-off-by: Greg Farnum <gfarnum@redhat.com>