p2p/discover: unlock the table during ping replacement

Table.mutex was being held while waiting for a reply packet, which
effectively made many parts of the whole stack block on that packet,
including the net_peerCount RPC call.

p2p/discover/udp_test.go

@@ -167,7 +167,7 @@ func TestUDP_findnode(t *testing.T) {
 	for i := 0; i < bucketSize; i++ {
 		nodes.push(nodeAtDistance(test.table.self.sha, i+2), bucketSize)
 	}
-	test.table.add(nodes.entries)
+	test.table.stuff(nodes.entries)
 
 	// ensure there's a bond with the test node,
 	// findnode won't be accepted otherwise.