Reset backoff to 10s (ethereum#6)

For whatever reasons the miners would appear to deadlock when this backoff time was 7s, 10s has been working reliably for a couple weeks.

p2p/dial.go

@@ -32,7 +32,7 @@ import (
 const (
 	// This is the amount of time spent waiting in between
 	// redialing a certain node.
-	dialHistoryExpiration = 7 * time.Second
+	dialHistoryExpiration = 10 * time.Second
 
 	// Discovery lookups are throttled and can only run
 	// once every few seconds.