p2p: seed_mode sometimes fails to send pexAddrsMessage before disconnecting gracefully

[ebuchman]

When SeedMode is true, the PexReactor sends a batch of addresses and calls StopPeerGracefully. It seems sometimes the peer ends up disconnecting before these messages are delivered, which makes it difficult for a new node to join the network.

It looks like we don't call .flush() in the MConnection.OnStop(). I tried adding that on the seed node, but even still the new node sometimes fails to receive any addresses from the seed.

[phymbert]

@ebuchman StopPeerGracefully yet implemented. bcStatusResponseMessage is processed before pexAddrsMessage. Connection is closed by seed peer before pexAddrsMessage arrived in pex channel...

I did a dirty fix that seem to work in pex_reactor.go:

if r.config.SeedMode {
			r.SendAddrs(src, r.book.GetSelectionWithBias(biasToSelectNewPeers))
			go func() {
				time.Sleep(5 * time.Second)
				r.Switch.StopPeerGracefully(src)
			}()
		}

I will try to submit a PR to handle shutdown gracefully properly.

[ebuchman]

@phymbert you solution was implemented in #2797. But see #2802 for my attempt to do it more correctly.