Align APIs of Database and in-memory backends

[wdanilo]

This task is automatically imported from the old Task Issue Board and it was originally created by Radosław Waśko.
Original issue is here.

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We have some methods that are defined only on Database or In-memory Column/Table.

There are some methods which only make sense on one of them, but such API disparity will confuse users - as switching backends will lead to method resolution errors which is not quite expected.

We should align the APIs, possibly just throwing Unsupported_Database_Operation_Error_Data for stuff that cannot be done in the Database backend without materializing - such an error is much clearer than the method resolution error.

I'm not sure how to handle Table.read - it doesn't make much sense to exist on the in-memory table, but maybe for compatibility it should just work as identity? i.e. Table.read self = self. This way switching to in-memory backend from Database won't break workflows that were materializing in some places. IMO it makes sense as then read will have the functionality of ensuring the returned table is in-memory (so doing nothing if it was already).