OPENDJ-1441 (CR-4303) Persistent searches on external changelog do not return changes for new replicas and new domains

Fixing a deadlock situation with ExternalChangeLogTest.
When removing domains, the cursors were not properly closed by ChangeNumberIndexer.run() before calling Object.wait().
Relied on the fact that DBCursors are not thread safe so if client code calls MultiDomainDBCursor.removeDomain(), then the cursor is immediately removed.
I did not see the problem before because unit tests on trunk execute by default with file based changelog db.


ChangeNumberIndexer.java:
In run(), simplified the code that removes the domain cursors.

CompositeDBCursor.java:
Removed removeNoLongerNeededCursors() and removedCursorsIterator() + extracted method removeCursor() from it.
Added another removeCursor() which also removes from the exhaustedCursors.

MultiDomainDBCursor.java
Consequence of the change to CompositeDBCursor.
Removed removeDomains field and removedCursorsIterator().
In removeDomain(), now call CompositeDBCursor.removeCursor().

DomainDBCursor.java:
Consequence of the change to CompositeDBCursor.