kv: verify RPC responses
kv: verify RPC responses

Just logs for now. I didn't observe impact on performance.

Closes #4799.

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