Merge pull request #1010 from tweise/hds

Hds
Merge pull request #1014 from tweise/hds

Move WAL interfaces
Merge pull request #1015 from tweise/hds

Add test for get
Merge pull request #1016 from tweise/hds

Relocate file access mock to test package.
HDSFileAccess unit test.
For now, for TFile and HFile implementation
Changes to make test pass
Merge pull request #1020 from tweise/hds

Add default key comparator.
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.