FC-38 System events and handlers for blue-green deployment

Handlers for new system events have been added, but they do not have any action yet.
Also corrected an issue where rate limiter was applied for ignored messages.

Signed-off-by: Steffan Westcott <steffan.westcott@clicktravel.com>
This is a work in progress. As noted in the commit comment, the purpose of this commit is to establish the system event and handler classes. None of the new event handler classes have any action i.e. they do actually do anything on handling the event.