implement API and handlers for snapshot comparison

Two new interfaces:
-- getting all snapshots
-- comparing two snapshots

Also some complicated heuristics for snapshot comparison

Change-Id: I0c4622b03a71970bb02251ebe490d5f315abb0e8
Reviewed-on: http://review.couchbase.org/33759
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>