FLUME-1778: Upgrade Flume to use Avro 1.7.3
FLUME-1778: Upgrade Flume to use Avro 1.7.3

(Hari Shreedharan via Brock Noland)

tmgstevens/flume,szaboferee/flume,iaiengxia/flume,szaboferee/flume,tinawenqiao/flume,tinawenqiao/flume,iaiengxia/flume,tmgstevens/flume,tmgstevens/flume,iaiengxia/flume,tinawenqiao/flume,szaboferee/flume,tmgstevens/flume,szaboferee/flume,iaiengxia/flume,tinawenqiao/flume