ceph-fuse: Avoid doing handle cleanup in dtor

ceph-fuse: Avoid doing handle cleanup in dtor

The CephFuse::Handle class needs the client
pointer to be valid for finalizing, so don't finalize
in the destructor (which doesn't get called till the
fuse handle leaves scope), instead use a finalize method that
gets called explicitly before the client pointer is freed.

Signed-off-by: Sam Lang <sam.lang@inktank.com>