[DDC-3160] Alternate fix for DDC-2996 bug

The fix implemented for DDC-2996 seems to have broken quite a bit of code outside of Doctrine (for instance, the popular DoctrineExtensions project).

I'm not too well versed in Doctrine internals, but looking at the fix, I found it odd that an entity would live in both the insertion and update tracking in the unit of work.

This implementation should fix the case described in DDC-2996, while not creating any side effects with objects that are scheduled to be inserted.

I've ran this test with the Doctrine test suite and have also tested this change with the DoctrineExtensions project; unless I'm doing something wrong, it seems to have good results.

Again, I'm not too well versed in the inner workings of Doctrine, but I did a little research, poked around some code, and came up with this.
Looks like there is already an OnFlushListener in DDC2790Test.php causing an error.