do ProtocolEvents fixing only when there is required fields missing in the requested schema

https://issues.apache.org/jira/browse/PARQUET-61
This PR is trying to redo the https://github.com/apache/incubator-parquet-mr/pull/7

In this PR, it fixes the protocol event in a more precise condition:
Only when the requested schema missing some required fields that are present in the full schema

So even if there a projection, as long as the projection is not getting rid of the required field, the protocol events amender will not be called.

Could you take a look at this ? @dvryaboy @yan-qi

Author: Tianshuo Deng <tdeng@twitter.com>

Closes #28 from tsdeng/fix_protocol_when_required_field_missing and squashes the following commits:

ba778b9 [Tianshuo Deng] add continue for readability
d5639df [Tianshuo Deng] fix unused import
090e894 [Tianshuo Deng] format
13a609d [Tianshuo Deng] comment format
ef1fe58 [Tianshuo Deng] little refactor, remove the hasMissingRequiredFieldFromProjection method
7c2c158 [Tianshuo Deng] format
83a5655 [Tianshuo Deng] do ProtocolEvents fixing only when there is required fields missing in the requested schema