Current commit redesigns distributed INSERT command. Previously we
dispatched insert SQL command to the storages. If INSERT could be
done locally (without building a virtual table on the router) we
used SQL "bucket_id(<string>)" function to recalculate buckets
on the starage via SQL.

This approach had the main disadvantage - it worked only with a
"bucket_id" SQL function that had a single argument as a parameter.
An attempt to support multiple parameters of different types (tuple
columns as we plan to implement for Picodata engine in future) faced
serious technical problems.
Also, the old implementation had performance issues:
- we created temporary spaces on the storage even for a single VALUE
  insertion
- we always dispatched VALUES to the storage to build a virtual table
- the resulting SQL was too verbose (as we produced a subquery under
  INSERT node)

It was desided to get rid of the old approach and migrate to non-SQL
insertion. It means that a new type of motion was introduled - local
segment. It allows us to build a virtual table on the storage in
Rust memory, then using space API transform and insert collected tuples
directly into the target space within a single transaction. Also we
can materailize VALUES (if they contain only constants) on the router
and get rid of the redundant network transmission over the network.