FCREPO-1024 with multi threaded unit test

This change is based on the earlier pull request (by damkjaer) for the FCREPO-1024 issue. It adds unit testing of two scenarios where threads can clash in DefaultDOManager.getIngestWriter, if they are not protected by synchronization and PID write locking.

The test shows that the method does not need to be synchronized if the PID write lock is taken before calling objectExists.

Removing synchronized on the method can greatly increase performance when ingesting objects with multiple threads