AVRO-4076: [java] Parse Fields before adding to the context.

[jamespfaulkner]

What is the purpose of the change

This pull request fixes the issue identified by AVRO-4076.

This is potentially not the best way to do this but I think it at least demonstrates what I think the issue is.
In Schema, we put the result into the context, (ultimately leading to the throw). But we haven't loaded the fields at that point, causing the equality check to fail even if the schemas do match.

Unfortunately I had to change the equals in Protocol.java in order to get the tests to pass. The order of schemas returned by ParseContext.resolveAllSchemas has slightly changed. I'm not sure that the order matters, but if I change ParseContext.resolveAllSchemas to return an unordered collection I get failures in TestIdlReader.runTests.

Verifying this change

This change added tests and can be verified as follows:

• TestSchema::testParseMultipleFile has been updated to include the redefined Schema scenario.

Documentation

• Does this pull request introduce a new feature? - no
• If yes, how is the feature documented? not applicable