non-sequential-migrations: Encode additional information in schema files

Created by: efritz

Read metadata within migration files that self-declare their parent (which will, at this stage, be dictated by sequentially increasing numbered files). Add this to validation and the migration addition utilities.

Additional validation should include a comparison between file names and self-declared parents in metadata, as well as ensuring that the migration parentage information forms a single-source directed acyclic graph.