69457338 http recent

Implements [#69457338](https://www.pivotaltracker.com/story/show/69457338): return recent (aka "dump") logs over HTTP instead of WebSocket. This makes it easier to get through HTTP-only proxies, and simplifies the protocol (since WebSocket isn't needed when not streaming).

The logs are returned in an HTTP response with ContentType `multipart/protobuf`, which consists of the original ProtocolBuffer messages (as were previously sent over WebSocket).

To ensure backwards compatibility, the new behavior is present at the "/recent" endpoint, and the original behavior at the original "/dump" endpoint.

Along the way, we've also upgraded the WebSocket connection between TrafficController and Loggregator to use standard ping/pong frames instead of non-standard keep-alive messages.