OPENDJ-1618 (CR-5155) External changelog not purged after the purge delay

The ChangelogDBPurger is smart: when changes exist in the DB, it sleeps until the oldest change can be purged.
The problem is that when admin changes the purge delay, the ChangelogDBPurger is not woken up to verify if the oldest change can be purged with the purge delay.
This fix wakes up the ChangelogDBPurger when the purge delay changes.


FileChangelogDB.java, JEChangelogDB.java:
In setPurgeDelay(), ensured an existing ChangelogDBPurger is woken up by a change to the purge delay.
Made purgeDelayInMillis volatile + In ChangelogDBPurger.run(), inlined jeFriendlySleep() to ensure the wait time is computed from the very latest purgeDelayInMillis.