[helper] Validate things more strictly under all versions of Avro.

[srramach]

The goal of the compat helper is to bridge the gap when users move from older Avro versions to newer ones. In general, older Avro is less strict about various things. For example, default values for schemas are not validated in Parser.parse() or new Schema.Field(); getProp() doesn't throw for non-existent properties; etc.

Since newer Avro is more strict about these things, we can help users by validating these things, even under older versions of Avro. This will let users catch issues earlier (before they actually make the switch to newer Avro).