WL#4603: Foreign Keys: implement cursor API for foreign key support

Provide a default implementation of the storage engine API for foregn key
support. The API itself is specified in WL#3288.
A default implementation can be used for engines that don't re-implement
the abstract interface specified in WL#3288.

Create an instance of storage engine handler by means of handler::clone() for
each foreign key cursor. Use handler::index_init(), handler::index_next() and
handler::update_row(), handler::delete_row(), etc. to implement cursor
functionality.