OPENDJ-1231 (CR-2724) Make the Medium Consistency Point support replica heartbeats and replicas shutting down


Replica heartbeats are now making the medium consistency point move forward.
This brings trunk implementation to be on par with OpenDJ 2.6.0.


ReplicationServerDomain.java:
In getEligibleCSN(), used ReplicationDomainDB.getDomainLastAliveCSNs().
Removed field ctHeartbeatState + methods getChangeTimeHeartbeatState() and storeReceivedCTHeartbeat(), all superseded by ChangeNumberIndexer.

ExternalChangeLogTest.java
Consequence of removing ReplicationServerDomain.getChangeTimeHeartbeatState().
Improved the code readability with waitOpResult() + moved calls to waitOpResult() inside searchOnChangelog().


ReplicationDomainDB.java, JEChangelogDB.java:
Added and implemented getDomainLastAliveCSNs().
In getDomainOldestCSNs() and getDomainNewestCSNs(), improved javadocs.

ChangeNumberIndexer.java:
Remain lastSeenUpdates field to lastAliveCSNs + improved javadoc.
Added getDomainLastAliveCSNs().


MultiDomainServerState.java
Added getServerState(DN).