Fix closing of sessions on mgm server shutdown.


ndb/include/mgmcommon/ConfigRetriever.hpp:
  Add disconnect();
ndb/src/common/mgmcommon/ConfigRetriever.cpp:
  Add disconnect() call so we can disconnect from the mgm server before it shuts down
  (if we are a mgm server).
ndb/src/mgmsrv/main.cpp:
  Close our ConfigRetriever connection first.
  
  Stop sessions, and wait for them to stop. (previously we didn't wait, this was
  causing core dumps on shutdown with failed nodes).