Additional fix for OPENDJ-875: Use of hostnames in replication protocol causes failover problems

* fix unit test regression introduced by previous commit. RS candidate filtering was too aggressive and assumed that there would only be one RS per JVM, which is not the case in the unit tests

* also use a thread safe set implementation for the local RS ports in order to avoid potential race conditions in unit tests (this may be the cause of occasional RS load balancing unit test failures).