A schema key collision will cause silent failure in bootstrapping

[mtnygard]

Description

If the user's schema includes a key and version that happens to match those in Simulant's built in schema (or any other conflict between files, for that matter) then the "migrations" code in database.clj will silently filter out attribute definitions.

This will lead to user confusion and frustration, because no error or warning message is produced and yet the user's schema definition will not be there.