OPENDJ-2543: ChangelogException while doing modrate on PDB instance.

Some cursor continue to be referenced in the openCursors list while they
are actually closed. This caused IOException during head log file
rotation when disableOpenedCursorsOnHead() tried to get state of those
closed and still referenced cursors.
By acquiring shared-lock on cursor.close() we ensure that the close()
and the removal of the cursor from the openedCursors lists are performed
atomically.