Override is broken for `guid` features

[mvalipour]

What

The part of code that checks overrides for determinations, seem to always look into the id -- while (the features bucket_type is guid) one would the keys on overrides to represent guids.

Solution

There is an easy solution of deciding to use id or guid here -- however backwards compatibility is a big issue here. Two ways I can imagine we get around this:

1. Do a major release and warn about the breaking change.
2. Check for both id and guid -- bit hacky but in reality these two never clash, so... we should be fine?

Thoughts?
/cc @jphastings