CBIDXT-140: Send Snapshot messages downstream.

Change-Id: Ic2b8b24f15f6cdd072641fb4f8770d4109f934b6
Reviewed-on: http://review.couchbase.org/40408
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>