kubo/client/rpc: way to signal node stopping

[magik6k]

Something like api.Closing() <-chan struct{}, to have a way to tell when daemon is shutting down, etc.

[Stebalien]

api.Context()?

[magik6k]

That works (and as a bonus we can implement the Context interface to return detailed cause through ctx.Err())

[Stebalien]

We need to be very careful about implementing the context interface to avoid blowing up the number of goroutines. If we do that, we should immediately wrap it in a cancelable context.

See https://golang.org/src/context/context.go#L257.