Payloads in events

[mezis]

There's a growing number of use cases where the "pure RESTful" approach of only sending events on the bus doesn't work (e.g. high-frequency updates where the subscribers might not manage to fetch intermediary representations).

Add support for Routemaster to accept and transmit resource representations in JSON.

Implementation notes:

• Auto-compress event data over 5kB.

[mezis]

Moved to https://deliveroo.atlassian.net/browse/IP-426