Flatten Junctions with the same nature

[beikov]

I just realized that we currently produce a SqmAndPredicate/SqmOrPredicate/Junction for every and/or operator which leads to deep AST trees.

We already manage a list of predicates in Junction, but it only ever receives 2 elements. What do you think about flattening the junctions instead to avoid these deep AST trees?

[beikov]

Since nobody objected, I'll consider this as agreement with the approach I described.