discovery+server: make waiting proofs persistent

In this commit waiting proofs array have been replaced with persistant
boltd storage which removes the possibility for the half proof to be
lost during half proof exchange.

server.go

@@ -254,6 +254,7 @@ func newServer(listenAddrs []string, notifier chainntnfs.ChainNotifier,
 		SendToPeer:       s.sendToPeer,
 		TrickleDelay:     time.Millisecond * 300,
 		ProofMatureDelta: 0,
+		DB:               chanDB,
 	})
 	if err != nil {
 		return nil, err