Infinite loop/recursion when scanning deeply nested Avro file.

When scanning a deeply nested Avro file Impala gets into an infinite loop/recursion and the query hangs. The query cannot be cancelled and will continue to take 100% of one CPU core. The only remedy is to restart the impalad.

Skye, I had applied Patch Set 4 of your Avro scanner CR.

Steps to repro:
1. Copy the attached Parquet data file to a local dir
2. Copy the file somewhere into a new HDFS dir (assuming /test-warehouse/max_depth/ below)
3. In Impala, create a Parquet table using that file:

create external table max_depth_parquet
like parquet '/test-warehouse/max_depth/max_depth.parq'
stored as parquet
location '/test-warehouse/max_depth/

4. In Hive, create an Avro table from that Parquet table:

create table max_depth_avro stored as avro as select * from max_depth_parquet;

During my initial investigation I found the following:
The query hangs in AvroSchemaElement::ConvertSchema() called from HdfsScanNode::Prepare().

I added some logging in AvroSchemaElement::ConvertSchema() to print the pointers of traversed child elements, and there appears to be a cycle because the pointers traversed repeat after some number of recursive calls.