MB-7629: Add structures and operations for view btree values

Change-Id: I488b2088cc3b151d85da4db7126cac60bba7dca7
Reviewed-on: http://review.couchbase.org/25500
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>
Tested-by: Fulu Li <fulu@couchbase.com>