Revisit Shutdown Behavior

[hannahhoward]

We have pretty terrible patterns for shutting down all over the code base.

Currently, you pass a context to Graphsync when you start it and the only way to shutdown is just cancel the context and hope all the go routines get cleaned up (I think they do but...)

Meanwhile, we actually have Shutdown methods for lots of the components but all they do is shut down an internal context... and they're not even exposed publicly.

Per @mvdan :

If we went with the "we'll eventually want to block until all request goroutines are stopped" route,
then I think these methods should  gain a context parameter, too. That way, I can say "graceful 
shutdown for 10s to stop accepting requests, and after that, kill all requests which aren't done
yet". This is how net/http does it: https://pkg.go.dev/net/http#Server.Shutdown

We should... have a way to shutdown that confirms all the go routines are actually closed.