Fixed an issue where a NPE could be thrown when the server processes a mod DN operation with a new superior entry thats in the subtree to be moved. An informative error message is added so the user can fix the mistake. 

This patch also enables the debug logger to print the cause of an exception message as well as all the stack frames by default. It also removes the deadlock retry configuration parameter from the JE backend configuration definition since deadlocks can no longer occur in the JE backend.

Fix for issue 3236