Allow consumers to accept upr snapshot markers as a no-op

When a consumer receives a snapshot marker we may or may not want
to actually manipulate the checkpoint manager on the consumer,
but since we are not sure exactly what we want to do let's just
return ENGINE_SUCCESS in order to unblock cluster replication work
from moving forward.

Change-Id: If503283b527b711dd9f32376814a51ae569d2df5
Reviewed-on: http://review.couchbase.org/32380
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>