runtime: selectgo need not  lock all the channels involved in the select 

i read the select function:
```
func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
   ...
	// lock all the channels involved in the select
	sellock(scases, lockorder)

}
```

why we should lock  all the channels?
 i think lock one by one  when loop may be better , because it will quickly