Run information_schema queries only in coordinator

This fixes an issue where queries over information_schema could run in
any of the workers, resulting in inconsistent metadata due to caching
(e.g., "describe <table>" might show 0 columns for a table that had
recently been deleted)

As part of this change, the split scheduler has been updated to pick
the coordinator for processing coordinator-only splits regardless
of the value of the includeCoordinator flag