Always persist IndexConfig

[asutula]

Adds tests for adding indexes as well

fixes #285
fixes #284

[sanderpick]

LGTM! 💣

[asutula]

One question I have:

Are there any consequences with the user overwriting an existing index and changing Unique from false to true, for example?

[asutula]

That all kinda makes my head hurt. For sure there could be issues once we support back-indexing already existing data. But since that's not yet the case, does switching the value of Unique have any consequences from the time it is switched going forward?

[carsonfarmer]

There are no knock-on effects to changing an existing index like that @asutula, it will just no longer check if a key is unique. The only place that the 'uniqueness' is persisted is in the index definition. The indexed data itself doesn't care about this directly, just the index is responsible check checking this.

[carsonfarmer]

The only consequences of course, are that if Unique was false previously, and then set to true, those past values that may or may not be unique will no longer be checked, only new data will be checked for uniqueness. So this won't throw, but it might be unexpected behavior. Hence why something like #286 would be good here as well.

[carsonfarmer]

Nice dude, I hope this, plus something like #290 will increase usability of this stuff!