Add default key comparator.
HDSFileAccess unit test.
For now, for TFile and HFile implementation
Changes to make test pass
test utilities
Merge pull request #1020 from tweise/hds

Add default key comparator.
random write test
Merge pull request #1021 from siyuanh/hds1

Fix bugs and add unit test for TFile/HFile FileAccess impl
Merge remote-tracking branch 'upstream/hds' into hds
Compare using offsets and length.
Merge pull request #1022 from tweise/hds

random write test
Support sharing comparator with memory store.
Test custom comparator with HFile.
Merge pull request #1026 from tweise/hds

Support sharing comparator with memory store.
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
Merge branch 'hds' of github.com:DataTorrent/Malhar into hds
Reuse blockreader