Add /debug/stopper endpoint
Add /debug/stopper endpoint

```
0xc8202eb570: 1 tasks
1      server/node.go:700
```

Closes #5231.

sunya123/cockroach,cuongdo/cockroach,vivekmenezes/cockroach,glycerine/cockroach,nieyy/cockroach,maxlang/cockroach,maxlang/cockroach,glycerine/cockroach,maxlang/cockroach,petermattis/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,veteranlu/cockroach,paperstreet/cockroach,sunya123/cockroach,sunya123/cockroach,vivekmenezes/cockroach,maxlang/cockroach,paperstreet/cockroach,nieyy/cockroach,cockroachdb/cockroach,bg451/cockroach,vaibhavarora/cockroach,sunya123/cockroach,cockroachdb/cockroach,petermattis/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,bg451/cockroach,glycerine/cockroach,glycerine/cockroach,veteranlu/cockroach,cockroachdb/cockroach,veteranlu/cockroach,arjun4084346/cockroach,sunya123/cockroach,vivekmenezes/cockroach,cuongdo/cockroach,paperstreet/cockroach,veteranlu/cockroach,petermattis/cockroach,sunya123/cockroach,cockroachdb/cockroach,petermattis/cockroach,petermattis/cockroach,paperstreet/cockroach,glycerine/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,nieyy/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,nieyy/cockroach,bg451/cockroach,maxlang/cockroach,vivekmenezes/cockroach,glycerine/cockroach,nieyy/cockroach,vivekmenezes/cockroach,maxlang/cockroach,paperstreet/cockroach,bg451/cockroach,cockroachdb/cockroach,glycerine/cockroach,nieyy/cockroach,bg451/cockroach,cockroachdb/cockroach,arjun4084346/cockroach,cuongdo/cockroach,cuongdo/cockroach,petermattis/cockroach,cuongdo/cockroach,nieyy/cockroach,vaibhavarora/cockroach,glycerine/cockroach,petermattis/cockroach,sunya123/cockroach,vaibhavarora/cockroach,cuongdo/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,bg451/cockroach,vivekmenezes/cockroach,cuongdo/cockroach,veteranlu/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,nieyy/cockroach,sunya123/cockroach,cuongdo/cockroach,petermattis/cockroach,bg451/cockroach,paperstreet/cockroach,maxlang/cockroach,cockroachdb/cockroach