Refactor tiered schema code to follow conventions

In the initial POC the schema support for tiered was put into its own directory and file, tiered/tiered_schema.c. This is understandable for keeping the POC self-contained, but is inconsistent with the rest of the codebase.

This is a small ticket to incorporate the existing tiered schema code into the schema files instead. This is only a code movement and should not be functionally different.