vpn-connection: don't dispose the proxy before we get the disconnect response

vpn-connection: don't dispose the proxy before we get the disconnect response

The return_method would be rejected by the dbus-daemon when the NM drops
its match, resulting in an ugly message in the log:

  method call time=1458301860.187048 sender=:1.267 -> destination=:1.276
    serial=5302 path=/org/freedesktop/NetworkManager/VPN/Plugin;
    interface=org.freedesktop.NetworkManager.VPN.Plugin; member=Disconnect
  method call time=1458301860.187054 sender=:1.267 -> destination=org.freedesktop.DBus
    serial=5303 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus;
    member=RemoveMatch
     string "type='signal',sender='org.freedesktop.DBus',
                                   interface='org.freedesktop.DBus',
                                   member='NameOwnerChanged',
                                   path='/org/freedesktop/DBus',
                                   arg0='org.freedesktop.NetworkManager.libreswan.Connection_10'"
  method return time=1458301860.187061 sender=org.freedesktop.DBus -> destination=:1.267
    serial=1835 reply_serial=5303
  ...
  method return time=1458301860.195351 sender=:1.276 -> destination=:1.267
    serial=19 reply_serial=5302
  error time=1458301860.195361 sender=org.freedesktop.DBus -> destination=:1.276
    error_name=org.freedesktop.DBus.Error.AccessDenied reply_serial=19
     string "Rejected send message, 7 matched rules; type="method_return",
             sender=":1.276" (uid=0 pid=26915
             comm="/usr/libexec/nm-libreswan-service --bus-name org.f")
             interface="(unset)" member="(unset)" error name="(unset)"
             requested_reply="0" destination=":1.267" (uid=0 pid=25724
             comm="/usr/sbin/NetworkManager --no-daemon ")"

Also, refcount the connection instance. While the proxy is alive, it
invokes singal callbacks that get the object as a parameter.