Performance problem: GetFrom and GetFromTo queries do not use an index

[drauch]

The fix implemented in #35 ruined the performance of the GetFrom and GetFromTo queries. It's still required to use CommitStamp, CheckpointNumber as ORDER BY clause in order that the corresponding index can be used.

Unfortunately I don't have the time for a pull request, but it should be a fairly minor change anyways, most effort will be to test that the index is now indeed used by the query.

Best regards,
D.R.