Upgrade to ROM v4

[v-kolesnikov]

Hi, folks!
I tried to update (#16) rom-csv to rom-core v4 and add some useful features as well.
For example - I added support for multi-file database as in rom-yaml - and it is working!
But there is one thing that bothers me - commands layer. I temporary disabled it because it was based on mutable datasets, that in itself bad.

And now I have some questions:

• Need we to support commands layer in CSV adapter?
• Is it OK to use mutations in datasets to support commands? (I think not)
• Which another rom-adapter will be better to use as conceptual example for the implemetation of commands layer? I look to rom-yaml and rom-mongo as most recently updated adapters.

@solnic @flash-gordon