FLUME-2401. Optionally compress backup checkpoint.

(Abraham Fine via Hari Shreedharan)

Conflicts:
	flume-ng-channels/flume-file-channel/src/main/java/org/apache/flume/channel/file/FileChannel.java
	flume-ng-channels/flume-file-channel/src/main/java/org/apache/flume/channel/file/FileChannelConfiguration.java