Fix to allow server adapter to return cleanly while IDE is starting

* The teiid server manager cannot be called on startup due to its init
  depending on ServerCore. Since adding this adapter as a trigger point
  on the Server tree, it can be called while the IDE has not yet started
  and certainly before ServerCore has initialised.

* Let the server adapter return null until the teiid server manager has
  been started. Since this adapter is called so often and the server tree
  refreshed so frequently it is better to return null rather than creating
  a waiting thread.