We have fixed some bugs about NULL and made equality propagation
use the infrastructure common with the other transformations.

Also, extended and refactored the tests.

Now we can eliminate buckets with multi-column (i.e. composite)
keys in equalities. Previous implementation was incorrect, so
we have to make an executor refactoring as well.

Add support join query to linker, now executor can execute join query with distribution conflict (plan has motion nodes). Extend text in some error

Join node deduces the distribution of its output tuple based on
the output tuples of its left and right children. The logic of
deduction was broken: if we swap the children the distribution
changed. So, current commit rewrites join distribution deduction.

Also we refactor some parts of the set_distribution() function.
Its code style is still bad, but now it is not so horrible as
before.

Planner may split some query into range of sub queries, where subquery
execution depends on results previous subquery. Then planner analyzes
query and if it finds similar situation he appends `motion` node to plan.
If executor finds `monion` node in execution plan, he executes
motion sub-query and saves result to virtual table, which linker will be used.