Replication of small import subselects and virtual schema tables for joins

Situation
Virtual schema access can be mapped to an import subselect subquery. Import subselects store data in a temporary table and use this in the original query context.

If the query context is a join queue where the import subselect is joined, small table replication could be applied to avoid expensive global joins. But, the optimizer never replicated import subselects whatever their size was.

Improvement
Small materializations from import subselects are now replicated in joins. The behaviour is consistent with the usage of other temporary tables. The subquery replication border is 100.000 rows by default.