Notes
-----

- Fixes multiple bugs that partially have been hidden deep in the system
  (e.g. cache id mismatch)
- Cleans up array level names to avoid confusion.
- Sets the border between query cache map and cache_map.
- Uses helper functions everywhere getCacheId(), etc.
- Defines a clear public external interface that external callers can use.
- By subclassing the DatabaseStatementPrefetch the fetch part is more
  stable (had warnings) and less code is duplicated.
- Fields no longer need to be specified for * queries - just omit the
  key in that case.

Added to README:

- Fixed keys -> columns and keys -> placeholders
- Clears cache_database on drush cc all