Implement fast index reads without data table check

- Fast, eventually consistent index-only queries
    - This will be the default mode. We'll support highly consistent reads
      with cross-check against data later (much of the code is already there).
    - We limit ourselves to queries on the index only so far. We might add the
      ability to select data that's not directly projected into the data table
      later (at the cost of one request per row request against the data
      table).
- Use _del field to exclude values that don't match any more
    - Add a test for this scenario, more tests needed
Make index updates properly idempotent

- Improve timestamp support for queries:
    - use placeholder syntax & Long conversion for the parameter
    - don't use it with the .if condition
- Make index updates properly idempotent by using proper timestamps for the
  respective queries:
    - data updates use the corresponding data row's timestamp
    - _del updates use the index update's timestamp, so that the most up to
      date index update wins
- Improve tests by checking the full results of get queries