Make test case work
WAL cleanup and metadata management.

- Remove unnecessary WAL metadata, keep only head and tail pointers.
- Store WAL metadata as part of HDSBucketManager check-pointed state.
- Cleanup old WAL files once tuples from them are written to data files.
Merge branch 'hds' of github.com:DataTorrent/Malhar into hds
Merge pull request #1027 from tushargosavi/hds

Hds - WAL changes