Ticket #47400 - MMR stress test with dna enabled causes a deadlock

Bug description: Under the heavy add/delete posix user entries,
dna_update_config_event causes a deadlock.

Fix description: dna_update_config_event starts transaction
before updating the shared config entry to avoid the deadlock
situation.

https://fedorahosted.org/389/ticket/47400

Reviewed by Rich (Thank you!!)
(cherry picked from commit c6a72a50e6c948647b220e4a44978f2c9c7e8466)