TEIIDDES-1708: Simplify threads in TeiidServerContentProvider

* TeiidServerContentProvider
 * Removes the load element job and merges its functionality with the
   refresh thread. Improves the performance and code readability.

* TeiidServer[Manager]
* EventManager
 * Moves the stop listener notification functionality onto the teiid server
   manager. Reduces the number of events passed to listeners when a server
   is reconnected.