ceph-fuse: create finisher threads after fork()

The ObjectCacher and MonClient classes both instantiate Finisher
threads.  We need to make sure they are created *after* the fork(2)
or else the process will fail to join() them on shutdown, and the
threads will not exist while fuse is doing useful work.

Put CephFuse on the heap and move all this initalization into the child
block, and make sure errors are passed back to the parent.

Fix-proposed-by: Alexandre Marangone <alexandre.maragone@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 4fa5f99a40792341d247e51488c37301da3c4e4f)