Alternative to #963. Instead of using atomics, this patch ensures that all state
changes are performed on the main thread. It also:
* Threads the context through explicitly to make sure fields changed by the main
thread are never read by the handler goroutines. This isn't strictly necessary
but it makes it easier to reason about.
* Uses a callback to signal "on done". There may be cleaner ways to do this (I
hate callbacks), but this nicely decouples the peer handler from the main loop.
fixes#963
Marten Seemann
Aarsh Shah
Steven Allen