Fix forestdb snapshot range scanner to adapt with key format

The new jsoncollate key representation contains secondary key
as well as primary key. This introduces certain challenges in
implementing inclusion options for range query.

This commit fixes the current implementation to support only the
following fixed range scan scheme. The inclusion option is unsupported
for now and will be fixed later.

For any range scan with low and high key specified, index scan
will return results with rows >= lowkey and rows < highkey.

Change-Id: I7644efcc0d31a01314183ea05e62a959069f1750
Reviewed-on: http://review.couchbase.org/43469
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>