Add HTTP throttle code for event operations.

[Namphibian]

The vent operations will be used primarily by clients that synchronise data from the API to their local persistent storage.

The operations can be draining on the system thus it is proposed to use HTTP 429 Too Many Request as a potential cure for clients being a bit optimistic over a slow connection for example.

:)

[Namphibian]

Added a 429 code to event_collection.raml