Implement data deletion

[satta]

It should be possible to purge data from a database, selected via some matching indicator. An example would be deleting all observations from a specific sensor ID.

[satta]

This is probably a feature of the specific backend, let's only track the impact on the RocksDB backend here, since it's bundled with the balboa source.
One might think of adding a compaction filter that drops entries if their sensor IDs match a given list of 'deleted' sensor IDs, as well as making sure these never show up in scans when accessed outside of a compaction. This is probably the closest we can get without having to lock and delete everything sequentially.