Batch delete operations within single aggregate

[schauder]

For storing non trivial aggregates many operations need to get performed.
Those should be performed using JDBC batch operations to minimise database roundtrips.

There will be issues for the different kind of operations:

• Batch insert operations within a single aggregate. #1159
• Deletes (this ticket)

This builds on the verification of #1155
Note that this is about batching for a single aggregate, while #537 takes this concept across aggregates.

[ctailor2]

The delete operations already appear to be batched within a single aggregate to the same degree as for insert operations, which is batching per entity relation. For each of the entities related to a aggregate that is being deleted, the sql statement to delete the related entities is formed using its property path relative to the root entity and it is parameterized with the root id. This is the extent to which delete operations can be batched using java.sql.PreparedStatement and further batching of multiple delete sql statements together using java.sql.Statement would require significant new functionality to create fully formed sql statements from sql statement templates and their corresponding sql parameter sources.