OPENDJ-1440 On startup, change number can progress without waiting for any heartbeat from known replicas

On startup, if a replication server knows about several replicas, then it must wait to receive some sort of alive information for each of them before being able to move the medium consistency forward.
Changes or heartbeats received after replication server started are acceptable.
Likewise, changes that would have been received before the replication server stopped are also acceptable.

This was fixed on replication server startup, by initializing the lastAliveCSN for each known replica, with the oldest possible CSN (timestamp == 0).
Then when checking if the medium consistency can move forward, if no medium consistency is set, then the lastAliveCSN for each known replica must have a timestamp != 0.


ChangeNumberIndexer.java:
In canMoveForwardMediumConsistencyPoint(), call allInitialReplicasArePastOldestPossibleCSN() if the medium consistency CSN is not set.
Added methods oldestPossibleCSN(), allInitialReplicasAreAlive().

ChangeNumberIndexerTest.java:
In emptyDBTwoInitialDSs(), slightly modified the code to test current bug.
In startCNIndexer(), added the initial ECL enabled domains as a parameter.