Message broker using Server-Sent Events

[chungers]

This PR contains a simple message broker build on SSE or server-sent events as a simple implementation of publish subscribe broker that works with unix sockets.

Message subscribers for different topics are organized internally using a radix tree so that a message published to the topic /cluster/controllers/instance will be delivered to subscribers to topics like/cluster, /cluster/controllers or /cluster/controllers/instance.

This is not meant to be a heavy-duty message broker like MQTT but is small and simple enough to be embedded into the core plugin server runtime and allow the plugins to communicate using events.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "plugin-sse" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353728584
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.