GODRIVER-1224 fix race condition in topology.pool

Change-Id: Ib283caf6e786a0e0636a9e462c12a0e8c395dbcd

Author: iwysiu

x/mongo/driver/topology/pool.go

@@ -270,7 +270,7 @@ func (p *pool) makeNewConnection(ctx context.Context) (*connection, string, erro
 
 	c.pool = p
 	c.poolID = atomic.AddUint64(&p.nextid, 1)
-	c.generation = p.generation
+	c.generation = atomic.LoadUint64(&p.generation)
 
 	if p.monitor != nil {
 		p.monitor.Event(&event.PoolEvent{