GDA-5898 Fix race condition in BatonManager

* The removeFacade method modifies the leaseHolders map; the
  removeTimeoutLeases method reads it

* The two methods need to synchronize on the same object so they can't run
  concurrently

* removeTimeoutLeases was synchronizing on the wrong object (the
  leaseRefresher instance, not the BatonManager instance)