Maintain WAL metadata in operator state.
Use window-ids as log sequence numbers.
Merge branch 'hds' of github.com:DataTorrent/Malhar into hds

Conflicts:
	contrib/src/main/java/com/datatorrent/contrib/hds/HDSBucketManager.java
	contrib/src/test/java/com/datatorrent/contrib/hds/WALTest.java
Merge branch 'hds' of https://github.com/tushargosavi/Malhar into tushargosavi-hds

Conflicts:
	contrib/src/main/java/com/datatorrent/contrib/hds/HDSBucketManager.java
	contrib/src/main/java/com/datatorrent/contrib/hds/HDSFileAccess.java
Fix merge conflict.