OPENDJ-1453 Replica offline messages should be synced with updates

In r10840, the change to JE/FileChangelogDB.getCursorFrom(DN, ServerState) unnecessarily triggers a lot of calls to the underlying DB (JE or file based) to retrieve the ChangelogState.
As an optimization, keeping an in-memory version of the ChangelogState in synch with the on-disk version will help.


ChangelogState.java:
Now thread safe.
Added removeOfflineReplica(), isEqualTo().
Changed domainToServerIds from Map<DN, List<Integer>> to Map<DN, Set<Integer>>.
In getOfflineReplicas(), now return a MultiDomainServerState.

ChangeNumberIndexer.java:
Consequence of the changes to ChangelogState.


ReplicationDbEnv.java:
Added changelogState field, updated at the same time as the on-disk changelogstate DB
Added getChangelogState(), called by client code instead of readChangelogState().
Renamed readChangelogState() to private readOnDiskChangelogState().
Added stateLock field to sync updates to in-memory and on-disk changelog state.

ReplicationEnvironment.java:
Added changelogState field, updated at the same time as the on-disk changelogstate DB
Added getChangelogState(), called by client code instead of readChangelogState().
Renamed readChangelogState() to private readOnDiskChangelogState().
Renamed domainLock field to domainsLock.

replication.properties:
Removed now unused error message.

FileChangelogDB.java, JEChangelogDB.java:
Consequence of the changes to ChangelogState and ReplicationEnvironment/ReplicationDbEnv.


MultiDomainServerState.java, ServerState.java:
Added getSnapshot() for unit tests.

ReplicationEnvironmentTest.java, ReplicationDbEnvTest.java:
Consequence of the changes to ReplicationEnvironment and ChangelogState.
Used the fake server.