Add definitions for Tearing Reads, Forward/Back Index

Change-Id: Ice7ac9606630f89924fb37337ef4bc85eb7b3b3a
Reviewed-on: http://review.couchbase.org/34470
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>