Import validate improvement

[pofider]

The import validation doesn't check if entities are actually valid.

We could run the real import in the transaction and roll it back in the end, but I don't think this is a good idea writing to db when validating something.

The second thought was that we add collection.validate and rewrite current validations so this is possible.
However, this has the downside in that validating duplicated names won't work.

The last idea is that we create a new instance of a memory store (store.provider=memory) somehow and perform the import there. This should give us the best validation possible without touching physical DB.