ceph-fuse: Split main into init/main/finalize

ceph-fuse:  Split main into init/main/finalize

With the invalidate callback enabled for fuse, the Client::unmount
call requires the fuse channel and session objects remain for performing
the invalidate callbacks.  This patch splits the ceph_fuse_ll_main
call into init, main, and finalize functions, so finalization of the
channel and session objects can be done after the unmount completes.

The patch includes cleanup for the code in fuse_ll.cc to make it more
in the style of C++ and make use of the pimpl idiom to hide the fuse
structures within the CephFuse::Handle pimpl class.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>