Refactor validation support

[kburke]

The entity-api app.py contains methods which could also be helpful to schema_validators.py, such as query_target_entity(), and the code in app.py is fully integrated with caching. Refactor this to a utility module available throughout entity-api.

Also, there are "additional validations" done by entity type rather than in schema_validators.py, possibly due to the need for supporting methods described in the preceding paragraph. Migrate these to schema_validators.py, as appropriate.

[yuanzhou]

@kburke it seems this upcoming improvement falls under this category. We can use a schema validator rather than having the validations in app.py. A small step towards this goal.