Fix https://bugzilla.redhat.com/show_bug.cgi?id=901137 Server shutdown during reconnect/failover

3 small (roughly independent) changes:
- HornetQRAManagedConnection: skip manually closing sessions and rely on ClientSessionFactory.close()
  for it. In practice, we do this by just closing the connection.
- ClientSessionImpl: set inClose := true, before closing children.
- ServerLocatorImpl to call causeExit() on all its ClientSessionFactories BEFORE calling close.
setStopSignal()
use specific flag to cancel failover