OPENDJ-1430 Some changes are missing from the external changelog

Code cleanup in ECLServerHandler:
The eligibleCSN was used when the ECLServerHandler was responsible for inserting replicaDB changes into the changeNumber index DB.
To this end, the eligibleCSN was computed to provide a kind of "medium consistency point" ("kind of" because it was badly computed).
After OPENDJ-1174, it is no longer ECLServerHandler's responsibility to inserting replicaDB changes into the changeNumber index DB, so the eligibleCSN is now useless and this commit removes it.

In addition this commit enhances the toString() methods for a better readability.



ECLServerHandler.java:
Removed eligibleCSN field + refreshEligibleCSN().
Renamed computeNextEligibleMessageForDomain() to computeNextAvailableMessage() + simplified code.
In DomainContext, removed nextNonEligibleMsg field + removed isEligible(), debugInfo() and toString(CSN).
In dumpState(), made the debug string more readable.
In buildDomainContexts(), used DomainContext new ctor + extracted method newDomainContext() to build full DomainContext objects in one go + fixed a possible ConcurrentModificationException.
In DomainContext, made some fields final + added a ctor + made toString() more readable.

ReplicationServerDomain.java:
Removed getEligibleCSN() and isServerConnected(), now unused.

ReplicationServer.java:
Removed getEligibleCSN(), now unused.

ReplicationDomainDB.java, JEChangelogDB.java, ChangeNumberIndexer.java:
Removed getDomainLastAliveCSNs(), now unused.

ExternalChangeLogTest.java
Consequence of the changes above.