Add a way to mark records for removal.

When we use an incremental update it would be nice to be able to mark certains IDs for removal.

One way to do so would be to add a `deleted: True` attribute in their record. In that case kinto-wizard needs to detect it and perform a DELETE action.

Start by reading the repository's incremental update handling and how records are currently written to Kinto. Trace where record attributes are inspected and where DELETE actions are performed. Done means records marked with `deleted: True` are detected during incremental updates and removed by the resulting DELETE action, with coverage for that behavior.

Written by the indexing model from the issue text.