snapview-server: register a callback with glusterd to get
 notifications

* As of now snapview-server is polling (sending rpc requests to glusterd) to
  get the latest list of snapshots at some regular time intervals
  (non configurable). Instead of that register a callback with glusterd so that
  glusterd sends notifications to snapd whenever a snapshot is created/deleted
  and snapview-server can configure itself.

rebase of the patch http://review.gluster.org/#/c/8150/

Change-Id: Iee2582b1a823d50c79233a41cf2106f458b40691
BUG: 1143961
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/8767
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>