fix consistent high timeout err rate caused by prepareStatement (#892)

* fix timeout bug

* remove cache no matter which err returns

conn.go

@@ -711,8 +711,8 @@ func (c *Conn) prepareStatement(ctx context.Context, stmt string, tracer Tracer)
 
 	framer, err := c.exec(ctx, prep, tracer)
 	if err != nil {
-		flight.err = err
 		flight.wg.Done()
+		c.session.stmtsLRU.remove(stmtCacheKey)
 		return nil, err
 	}