ovsdb: test ovs-vswitchd for backward compatibility

ovsdb: test ovs-vswitchd for backward compatibility

Add test to make sure ovs-vswitchd fall back to use the
"monitor" method when connecting to an older ovsdb-server that
does not support "monitor2".

For testing backward compatibility, add an ovs-appctl command:
"ovsdb-server/disable-monitor2". This command will restart
all currently open jsonrpc connections, but without support for
'monitor2' JSON-RPC method for the new connections.

There is no corresponding enable command, since this feature is only
useful for testing.  'monitor2' will be available when ovsdb-server
restarts.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@ovn.org>