CBIDXT-279 scanner: Add support for inclusion option in range scans

Implemented forestdb snapshot scanner to perform the following
operations:

include_low=off:
 - Read and discard all equal keys=lowkey

include_high=on:
 - Read and send all equal keys=highkey

Change-Id: I3e1eb31cf7e1a79ecc0e9e5dcb843c14de7c3458
Reviewed-on: http://review.couchbase.org/43641
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>