no need for map - tx responses should arrive in order (Refs #237)
```
me: so we are executing them in order and receiving them in order and there is no way we could receive them out of order (due to network or something else), correct?
ebuchman: if we receive them out of order, ABCI is broken
ebuchman: so it is possible, if the ABCI server we're talking to is not implementing the spec
ebuchman: but that shouldn't justify us building a map
```