Add btree lookup comparator helper for handling special cases
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 <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Filipe David Borba Manana <e0fcd583df77ec6c08e6d1257619cc1216ddc994@gmail.com>
Tested-by: Filipe David Borba Manana <e0fcd583df77ec6c08e6d1257619cc1216ddc994@gmail.com>

couchbaselabs/couchstore,jimwwalker/couchstore,couchbaselabs/couchstore,abhinavdangeti/couchstore,vmx/couchstore,vmx/couchstore,t3rm1n4l/couchstore,vmx/couchstore,t3rm1n4l/couchstore,jimwwalker/couchstore,couchbaselabs/couchstore,jimwwalker/couchstore,t3rm1n4l/couchstore,abhinavdangeti/couchstore,abhinavdangeti/couchstore