bugfix: reorder fields in thrift struct caused writting nulls. fixed it by keeping track of which fields are being written in each level, and only write nulls when current level is finished in MessageColumnIO
rename var
remove fieldCount from marker
format
Merge pull request #298 from Parquet/bugfix_reorder_thrift_fields_causing_writting_nulls

Bugfix reorder thrift fields causing writting nulls
Support field renaming for Avro read schemas, by means of
field aliases.

Avro 1.7.6 is required since it fixes https://issues.apache.org/jira/browse/AVRO-1433
But note that this is only to allow the test to run correctly.
Add explicit blank namespaces to account for change in AVRO-1295 in Avro 1.7.5.