7329: It appears that UnixVolume -> Compare method is falling in infinite loop due...

7329: It appears that UnixVolume -> Compare method is falling in infinite loop due to the fact that EOF is not returned when reading an empty file.
Due to this, PutBlock for an EmptyHash is resulting in infinite loop when an EmptyBlock already exists in Keep.
Added a quick fix in the Compare method for unix volume implementation. We can try to see why EOF is not returned and put in a better fix,
but for the time being this helps with unblocking tests.

Added several tests for empty block. Most importantly, added an keep integration test with UnixVolume so that we have integration tests covering some
of these code paths. More tests are definitely called for in this intergration test for better code coverage.

Also did gofmt and golint updates.