Make QueryBuilder backend agnostic

[muhrin]

Currently the QueryBuilder is written with relational databases in mind, and in fact probabaly specifically with using either Django or SQLA. This is at odds with the rest of the entities in aiida.orm which could suppot any backend type which implements the general backend interfaces (found in aiida.orm.implementation).

There are a number of implications to this beyond just not being able to use the current implemetation should a new backend ever be added including that the methods in QueryBuilder are very long and come with a corresponding greater maintenance difficulty. This would be helped by splitting things up into the things general to the QueryBuilder and things related to relational databases.

Given that there is quite a bit of shared implenetation between Django and SQLA (given that SQLA is used for querying in both cases), I would be tempted to to have a superclass for both Django and SQLA that implements aiida.orm.implementation.BackendQueryBuilder.

Amongst other things the final implementation would:

• Have no sqlalchemy imports in aiida.orm.QueryBuilder
• Have no sqlalchemy imports in aiida.orm.implementation.BackendQueryBuilder
• Methods like (and calls to them) get_filter_expr, get_filter_expr_from_column and get_filter_expr_from_attributes would be moved out of aiida.orm.QueryBuilder and down to the implemetation level only because a general QueryBuilder should not even rely on there being an expression that performs the query (it may just ben an API call)

[lekah]

I'm not going to work on this since I see no improvement for the user experience and this issue would require a lot of work and time that I don't have. I've assigned @sphuber in case someone wants to take this up in the next coding week, or to close it at his discretion.

[giovannipizzi]

I think it's ok to close for now, and the be reopened once we want to extend beyond SQL(alchemy)