OPENDJ-1177 (CR-3314) Re-implement changelog purging logic

JEChangelogDB.java:
In ChangelogDBPurger.run(), changed code to:
- support the absence of change number index DB. To simplify matters, code assumes that ds-cfg-compute-change-number does not change during the life time of an RS.
- purged by using a CSN made up from the purge delay rather than using the previous cookie.
- sleep 500 millis if there are no changes to purge, or sleep till the next change to purge.
- gracefully shutdown without fuss in the logs.


JEChangeNumberIndexDB.java
In purgeUpTo(), return the oldest non purged CSN rather than the previous cookie + merged two branches of the code.

ExternalChangeLogTest.java:
Consequence of the change to JEChangeNumberIndexDB.
Extracted method assertECLLimits() from ECLCompatTestLimits() + added a loop inside it to let the code persist changes asynchronously from the test thread.

JEChangeNumberIndexDBTest.java:
Renamed testTrim() to testPurge().
In newReplicationServer(), enabled ds-cfg-compute-change-number.