client: register callback for fuse interrupt

libfuse allows program to reigster a callback for interrupt. When a file
system operation is interrupted, the fuse kernel driver sends interupt
request to libfuse. libfuse calls the interrupt callback when receiving
interrupt request.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
client: allow interrupting blocked file lock operation

This commit introduce two new types of setfilelock request. Unlike
setfilelock (UNLOCK) request, these two new types of setfilelock request
do not drop locks that have alread been acquired, they only interrupt
blocked setfilelock request.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
client: use atomic variable to track reference of MetaRequeset

this allow us to increase reference count of MetaRequest while not holding
the client_lock

Signed-off-by: Yan, Zheng <zyan@redhat.com>
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>