Add some brokers

This is for @nchapman, @zaach, @vladikoff and @ckarlof. These are the first brokers, see what you think!

feat(client): Add some brokers!

* Start to add brokers that know how to startup/complete flows.
* 3 brokers to start, fx-desktop, redirect, web-channel.
* Get rid of the Channels high level abstraction, these are used directly by the brokers if needed.
* Move some code from service-mixin and various channels to brokers.
* Update tests to make use of the brokers.
I'm scratching my head with all these helpers. Can this be consolidated to `this._searchParam('service') === Constants.FX_DESKTOP_SYNC`. Do we have reference to the relier here? Can it just be something like `relier.isDesktopSync()` or similar?