[0.94 UNIT TESTS] TestAssignmentManager fails frequently on CentOS 5

TestAssignmentManager#testBalance() fails pretty frequently on CentOS 5 for 0.94. The root cause is that ClosedRegionHandler is executed by an executor, and before it finishes, the region transition is done for OPENING and OPENED.

This seems to be just a test problem, not an actual bug, since the region server won't open the region unless it get's it from the assign call on ClosedRegionHandler.process().

I've seen that HBASE-6109 has a fix for this already, will just backport those changes. This is 0.94 only.