MB 7629: Add structures and operations for view btree keys

Change-Id: I939abd94f91a3e855c94b50dd8d736e358542df3
Reviewed-on: http://review.couchbase.org/25450
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Fulu Li <fulu@couchbase.com>