OPENDJ-1177 (CR-3278) Re-implement changelog purging logic

In JEReplicaDB, simplified the logic that handled the internal queue that is used before actually persisting UpdateMsg changes to the underlying Berkeley JE DB.
Simplified the publisher/consumer model (msgQueue.add() / msgQueue.remove()) by relying on a LinkedBlockingQueue and a semaphore, instead of many synchronized blocks and fields that cluttered this code.

Code review: Matthew Swift


JEReplicaDB.java:
Changed msgQueue from LinkedList to LinkedBlockingQueue.
Removed fields queueMaxSize, queueLowmark, queueHimark, queueLowmarkBytes, queueHimarkBytes, queueByteSize and replaced them all with queueSizeBytes Semaphore.
Removed clearQueue() and getChanges().
Added collectAllPermits().
Added immutable CSNLimits class to remove the need for synchronizing on oldest and newest CSNs.
In clear(), calling collectAllPermits() while holding the flushLock actually prevented flushing to happen because it also needed the flushLock. Calling collectAllPermits() does not require to hold the flushLock, and once this method returns the msgQueue should be empty anyway and all the changes should have been pushed to the DB (flush() first removes messages from msgQueue, then add to DB, then releases all permits). In effect, there is no need to synchronize on flushLock anymore.
In trim(), the check for queue being below low mark was wrong (comparing queue size with number of bytes): extracted isQueueAboveLowMark() and fixed its definition + removed synchronized (flushLock) because it is now deemed unnecessary.
In generateCursorFrom(), Removed unnecessary call to flush() because that method is also called from JEReplicaDBCursor ctor.

ReplicationDB.java:
In addEntries(), now return the total size of the persisted messages (return type was void).

JEReplicaDBTest.java:
In testTrim(), allowed the test to finish + made the code clearer.

replication.properties:
Added an error message for adding a change to the JEReplicaDB.