OPENDJ-1467 :  File Based Changelog must support replicas temporarily leaving the topology

[Note: real merge of all changelog.file package content to be done in one shot in
 a future commit]
 
* File-based changelog
 Store the offline CSN when a replica goes offline
 Read the offline CSN if present to build the changelog state at startup
 
 Update ReplicationEnvironment.java to manage read and write from/to storage
 Update ReplicationEnvironmentTest.java with more unit tests
 Update replication.properties with new messages
 
* File and JE based changelog
 Remove the offline CSN if present when receiving an heartbeat or an
  update message
  
 Update FileChangelogDB.java, JEChangelogDB.java, ReplicationDBEnv.java to
 manage online replica notification
 
* Other minor updates : renaming, comments