ISPN-4521 ReplicationQueueTest.testReplicationQueueMultipleThreads

random failures

* Updated test to use proper assertions
* Changed test to use eventually
* Changed test threading to use thread pool

https://issues.jboss.org/browse/ISPN-4521
It never worked that way before (you could have 0-numThreads start at the same time (we would need a cyclic barrier to guarantee all of them or 2 countdown latches).  Also the test from what I can see doesn't depend on them starting at the same time.