p2p/discover: bump maxBondingPingPongs to 16

This should increase the speed a bit because all findnode
results (up to 16) can be verified at the same time.

p2p/discover/table.go

@@ -25,7 +25,7 @@ const (
 	hashBits   = len(common.Hash{}) * 8
 	nBuckets   = hashBits + 1 // Number of buckets
 
-	maxBondingPingPongs = 10
+	maxBondingPingPongs = 16
 )
 
 type Table struct {