OPENDJ-1177 Re-implement changelog purging logic

Code review: Matthew Swift

Code fix after r10601.
Several problems shown by continuous integration:
* Deadlocks,
* Long waits

JEReplicaDB.java:
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.