gfapi: implement a minimial glfs_fini()

At the very least, we should PARENT_DOWN on the currently active
graph and disconnect ourselves from glusterd.

Further cleanups underway.

Change-Id: I9276686a84b0975b5ce272b4cbec1b80920d5c5c
BUG: 1004519
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5903
Tested-by: Gluster Build System <jenkins@build.gluster.com>