HTTP API worker's PUT features should be splittable into a separate worker process.

Because of design limitations of the current HTTP API (largely due to our AMQP Consumer base class not making it easy to create auto-deleting queues) it is not safe to run multiple instances of the HTTP API vumi_app worker.

As a side-effect all handling of `PUT` replies by 3rd party apps also are currently funnelled through this single worker.

The concurrency limitations only apply for the streaming GETs, not for any of the PUT replies. We should be able to run multiple HTTP API workers that only accept PUTs on multiple servers to be able to load balance the replies or outbound messages coming from 3rd party applications.

:cake: and :cookie: for @vgeddes for pointing this out.