kv: Replace rpcError with roachpb.SendError
kv: Replace rpcError with roachpb.SendError

We don't need an extra type just to make a retryable util.Errorf, and
SendError has the same semantics that are desired here.

Closes #6410

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