Fix #426.

It was a bug in the test: we were sending back a return with the
question ID from the bootstrap message, instead of call A. The vast
majority of the time these end up being both zero because the former
gets returned to the pool, but every so often they're different,
triggering the bug.

rpc/level1_test.go

@@ -249,7 +249,7 @@ func testSendDisembargo(t *testing.T, sendPrimeTo rpccp.Call_sendResultsTo_Which
 		err = pogs.Insert(rpccp.Message_TypeID, capnp.Struct(outMsg.Message), &rpcMessage{
 			Which: rpccp.Message_Which_return,
 			Return: &rpcReturn{
-				AnswerID: bootQID,
+				AnswerID: qidA,
 				Which:    rpccp.Return_Which_results,
 				Results: &rpcPayload{
 					Content: results.ToPtr(),