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
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