This commit introduces a new Subscription type, which is synonymous with
ethereum.Subscription. It also adds a couple of utilities that make
working with Subscriptions easier. The mot complex utility is Feed, a
synchronisation device that implements broadcast subscriptions. Feed is
slightly faster than TypeMux and will replace uses of TypeMux across the
go-ethereum codebase in the future.
This fixes an issue where the following would lead to a panic due to a
channel being closed twice:
* Start mux
* Stop mux
* Sub to mux
* Unsub
This is fixed by setting the subscriptions status to closed resulting in
the Unsubscribe to ignore the request when called.
