Add btree lookup comparator helper for handling special cases

Btree lookup api has the option of doing full btree iteration
by adding a NULL lowkey in the lookup request keylist. Added a
comparator wrapper to handle this special case so that any of
the comparator implementations need not handle this special case.

Change-Id: I963dcab73da18cc2d4eb25e393581e8341452cf4
Reviewed-on: http://review.couchbase.org/31659
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>