AVRO-1268. Java: Extend support for stringables from reflect to specific.  Contributed by Alexandre Normand and cutting.

git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1461856 13f79535-47bb-0310-9956-ffa450edef68
AVRO-1259. Java: Improve Trevni's encoding of sparse columns.

git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1461896 13f79535-47bb-0310-9956-ffa450edef68