A transaction which inserts a child row may run concurrently with a
transaction which deletes the parent row for that child. One of the
transactions should be aborted in this case, or there will be an
orphaned child row.

Before committing an INSERT on a child row, a READ of the parent
row is triggered to confirm that the parent exists. Similarly, before
committing a DELETE on a parent row, a READ or SCAN is done to confirm
that no child rows exist. If the INSERT(child) and DELETE(parent)
transactions are run concurrently, their PREPARE and COMMIT ops may
interleave in such a way that both transactions commit. This occurs
because the triggered reads are done using CommittedRead locks, which
are not strong enough to prevent such error scenarios.

This is fixed by using the stronger lock mode of SimpleRead for both
triggered reads. The use of SimpleRead locks ensures that at least one
transaction will abort in every scenario of interleaved ops from
concurrent INSERT(child) and DELETE(parent) transactions.

Reviewed-by: Frazer Clement <frazer.clement@oracle.com>

Change-Id: Ifde79ff11ee1a3dac8d955490b9140cd07d3d242