OPENDJ-1177 (CR-3304) Re-implement changelog purging logic


After OPENDJ-1174, change number index DB is now populated eagerly (was populated lazily).
This means we can reap the benefits by changing how purging is done for changelogDB.
Previous purge process was driven by the replicaDBs: first purge replicaDBs then purge the change number index DB and it was causing lots of problems like stale CNIndexDB records for example.
New purge process is driven by the change number index DB: first purge change number index DB then purge the replicaDBs based on the oldest valid referenced record in the change number index DB.

Moved JEChangeNumberIndexDB purge thread to JEChangelogDB + made it responsible for purging the ChangeNumberIndexDB and all the ReplicaDBs.
In JEReplicaDB, thread is now only responsible for flushing, previously it was responsible for trimming and flushing which complicated the design and made it less efficient for both operations.



JEChangelogDB.java:
Added inner class ChangelogDBPurger.
Added fields  purgeDelay, cnPurger and latestTrimDate.
Extracted method startIndexer()
Removed getChangeNumberIndexDB(boolean) + associated code.
Reimplemented getDomainLatestTrimDate() and setPurgeDelay().

JEChangeNumberIndexDB.java:
No longer implements Runnable + removed run().
Removed fields trimmingThread, trimAge and replicationServer.
In ctor, removed ReplicationServer parameter.
Removed startTrimmingThread(), setPurgeDelay() clear(DN, AtomicBoolean).
Renamed clear(DN) to removeDomain(DN).
Renamed trim(AtomicBoolean) to purgeUpTo(long).
Added purgeUpToCookie(ChangeNumberIndexRecord).

JEReplicaDB.java:
Removed fields latestTrimDate and trimAge.
In run(), no longer call trim().
Removed getLatestTrimDate(), isQueueAboveLowMark(), setPurgeDelay() and getQueueSize().
Renamed trim() to purgeUpTo(CSN).
Made flush() private + reduced wait time on polling the msgQueue to speed up shutdown.
Added getNumberRecords() for unit tests.

JEReplicaDBCursor.java:
Since flushing is now eager, removed all calls to JEReplicaDB.flush().
Extracted methods closeCursor().

ReplicationServer.java:
Renamed getTrimAge() to getPurgeDelay() for consistency.

ReplicationDB.java:
Added getNumberRecords().

ChangeNumberIndexer.java, ChangeNumberIndexerTest.java:
Removed hacks due to old purging code.

ExternalChangeLogTest.java:
Called ChangelogDB.setPurgeDelay() instead of ChangeNumberIndexDB.setPurgeDelay(0).
Consequence of the changes to JEReplicaDB.
Removed method setPurgeDelayToInitialValue() that was not doing anything (JEChangeNumberIndexDB was always null).
In getCNIndexDB(), removed useless null check.

JEChangeNumberIndexDBTest.java:
Removed constants value1, value2, value3 which are invalid cookies.
Replaced them with fields previousCookie and cookies.
Added clearCookie() method.
In addRecord(), removed cookie parameter and build the cookie from the new fields.
Consequence of the changes to JEChangeNumberIndexDB.

JEReplicaDBTest.java:
In waitChangesArePersisted(), used JEReplicaDB.getNumberRecords() instead of JEReplicaDB.getQueueSize() + added parameters describing the number of expected records + the counter record window.
Replaced all calls to JEReplicaDB.flush() with calls to waitChangesArePersisted().
Consequence of the changes to JEReplicaDB.