Support immutable strings in AvroIndexedRecordConverter

If consumers are loading Parquet records into an immutable structure
like an Apache Spark RDD, being able to configure string reuse in
AvroIndexedRecordConverter can drastically reduce the overall memory
footprint of strings.

NOTE: This isn't meant to be a merge-able PR (yet). I want to use
this PR as a way to discuss: (1) if this is a reasonable approach
and (2) to learn if PrimitiveConverter needs to be thread-safe as
I'm currently using a ConcurrentHashMap. If there's agreement
that this would be worthwhile, I'll create a JIRA and write some
unit tests.
You can use the return value of the put to avoid a get - if null use valueAsString.