Transactions and purging

This package exposes an AbstractExecutor class and 3 concrete implementations for doctrine/orm, doctrine/mongodb-odm and doctrine/phpcr-odm.

The executors purge the database, then load the fixtures. The ORM implementation wraps these two steps in a database transaction, which provides a nice additional property: atomicity. Because of that transaction, the loading either succeeds or fails cleanly, meaning nothing is actually changed in the database if the loading fails. It delegates the purging to a separate class that can be configured to either use a TRUNCATE or a DELETE statement to empty tables.

Not all RDBMS have the capability to allow TRUNCATE statements inside transactions though. Notably, MySQL will produce the infamous "There is no active transaction" message when we attempt to close a transaction that was already implicitly closed.