storage: Simplify MVCCStats locking
storage: Simplify MVCCStats locking

We already had a method to do this.

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