1. Controller switchEvents queue should be priority based. The queue holds switch...

2. When switch is going to disconnect, SwitchHandler can clean up some internal states to speed up the process in order to avoid unnecessary messages backing up in the queue.
3. In SwitchManager, don't send additional port down notifications after node down event is received.
4. Fixed an issue where links are not recovered when the switch is reconnected to the controller.
5. Fixed TLS exception@java.security.ProviderException.

Change-Id: I4ba190a59f8d046df999401572d3c6822b15429b
Signed-off-by: Jason Ye <yisye@cisco.com>