remove options maxItemSizeKb

[reconbot]

Is your feature request related to a problem? Please describe.
It's a hard limit on DDB, it doesn't make sense to have it configurable. A major release is an easy time to drop it.

https://github.com/architect/dynalite/blob/main/db/index.js#L12

I'm not clear how correct this default limit of 400kb is, but nobody has complained. The docs do tell a story we could test against. However nobody has complained, so it's probably close enough.

Related we might need better validation but that's a separate issue.

[ryanblock]

I don't see anything from the legacy 2011-12-05 DynamoDB API that refers to configurable max item size, so barring any arcane knowledge @mhart may have to hand down on this I agree we can drop that setting in 4.0.

Related, it's lower priority but not a bad idea to add in that per-item overhead mentioned in that doc of 100b for indexing. The other features would be trickier to calculate since they haven't published how many bytes overhead are required for each feature.