connect traffic controller to doppler

[#75365570]

Signed-off-by: John Tuley <jtuley@pivotal.io>
metron message_aggregator: use stop event's app id

Real-world start events from router do not include application id, but stop
events do.

[Fix #78500164]

Signed-off-by: John Tuley <jtuley@pivotal.io>
cleanup doppler websocket_server

Signed-off-by: John Tuley <jtuley@pivotal.io>