XD-270 Hdfs file naming and rollover

This commit introduces and removes following items:
- removed spring-xd-hadoop/src/main/java/org/springframework/xd/batch,
  MarkP mentioned that this was copied from examples and not used.
- removed old hdfs sink stuff under spring-xd-hadoop/src/main/java/org/springframework/xd/hadoop,
  to be replaced with new implementation
- build.gradle changes for using spring-xd-extension-hdfs and spring-test
- rollover and naming system added to spring-xd-extension-hdfs
- HdfsOutboundChannelAdapter and supporting stuff around it re-written to use hdfs storage
- added/re-organized tests
- re-write of hdfs sink.xml to use new framework