Throw validation errors if uploads are too large

- [x] **total tm2z filesize** - should not exceed `750k` (to start)
- [x] **gzip bombtest** - error out if the stream out of gunzipping exceeds some num of bytes ... (5M? lower?)
- [x] **mapnik xml** - size, guessing `750k` is a good limit here

I feel like these values should not be hardcoded and should instead be configurable from the `unpacker` or other instances where `tilelive-vector` is used, any suggestions on how to best implement this?

/cc @yhahn