At the moment it seems that we are enough with two distribution
types: by key (segment) and not by key (any). So, remove all other
types as redundant.

Added a test case with subquery as well.

Now we can traverse only relational nodes in the tree in arbitrary
order.

It is required to traverse the chain of "and" boolean expressions.

At the moment we don't add tests as upcoming equality propagation
would implement enough tests for this scenario.

We don't want to mess with subqueries in projection as our query
set doesn't contain such queries. It also simplifies some
transformations (equality propagation for example).

This low-level function can be helpfull to construct rows with a
mix of references and constants.

Previously, row expressions (i.e. tuples) expected to have alias
nodes in the list. It is correct for relational node output, but
for selection filters and join conditions we don't need aliases.
So, add api to produce rows with and without aliases.

Previously, we didn't pass logaicl ID, that was incorrect we have
to generate ID first for the filter boolean expression, and then
pass it to the selection node as well.

Add a test for successfull Plan::add_union_all() run.

Also fix some documetation style