FLUME-1697: Update CHANGELOG after 1.3.0 RC0
FLUME-1697: Update CHANGELOG after 1.3.0 RC0

(Brock Noland via Brock Noland)

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