LU translation functions don't serialize

[shyamd]

The LU translator functions don't json serialize so the Store no longer dumps to json if you use one of them. I'll leave it up to you to decide how you want to deal with this. I'd think we might just have key type strings and the stores just translate to a standard internal format?

[dwinston]

I'm fine with e.g. assert lu_key in ['datetime', 'isoformat'] and mapping those str keys to the current function-pairs. I'm not interested in supporting other lu_field formats. I don't think any other formats are in use by anyone.

[shyamd]

fixed in 58b3e4e