engine: fix Iterator.{UnsafeKey,UnsafeValue} comments
engine: fix Iterator.{UnsafeKey,UnsafeValue} comments

paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach