event: address Feed review issues
event: clarify role of NewSubscription function
event: more Feed review fixes
* take sendLock after dropping f.mu
* add constant for number of special cases
event: fix subscribing/unsubscribing while Send is blocked
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.
