Change default LockManager back to nullLockManager.

* This was changed in 926db7c3bdcf8911b5314ffd5c1c7d0064d2734f as a workaround for a testing issue.
* See 8cbd679aac781bdd73038064261736a937645fc1 , which fixes the test harness in core .

Change-Id: Ib57cafee35a699ce875ee4bc8951781e1a37553a