Track unflushed changes?

[Syndesi]

Currently changes can be prepared via merge() etc., but if flush() is never called, these changes will not be persisted.

Proposed changes:

• Add method to query whether the element manager has outstanding changes.
• Add Symfony lifecycle event listener on response, which checks if there are outstanding changes, and if there are, write some notice in the logs.
• Think about a way to integrate a "have changes be left behind?"-check in integration tests, such that dangling / unpersisted changes get surfaced quickly.