[PeerList] Refactor Peer Start/Finish functions for Peers && Add finish closure to PeerLists

[willhug]

Peer Interface Old world:

StartRequest() (finish func())

Peer Interface New World:

StartRequest(Subscriber) // Subscriber is the subscriber that *Shouldn't* be notified
FinishRequest(Subscriber)  // Subscriber is the subscriber that *Shouldn't* be notified

PeerList old ChoosePeer func:

ChoosePeer(context.Context, *transport.Request) (Peer, error)

PeerList New ChoosePeer func:

// ChoosePeer should run the Peer.StartRequest and Peer.FinishRequest (through it's onFinish closure)
ChoosePeer(context.Context, *transport.Request) (p Peer, onFinish func(error), error)

This will allow the PeerLists to safely update their own internal state (within a lock) and remove the opportunity for a deadlock from the Notify.

[abhinav]

StartRequest(Subscriber) // Subscriber is the subscriber that *Shouldn't* be
notified
FinishRequest(Subscriber)  // Subscriber is the subscriber that *Shouldn't* be notified

That's a weird interface. StartRequest(Subscriber) sounds like the complete
opposite of that.

[willhug]

This is half finished now after @kriskowal 's change. The only change left is updating the ChoosePeer format to return the "on finish" method

[kriskowal]

Prioritizing for RC 3, @willhug