storage: Fix LastUpdateNanos becoming inconsistent with splits/merges.
storage: Fix LastUpdateNanos becoming inconsistent with splits/merges.

Because RangeStats are meant to be consistent the LastUpdateNanos
cannot be set using the wallclock on the local node. During a
split/merge the new replica uses a LastUpdateNanos value from an
existing consistent value.

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