[FEATURE] Option to skip writing filled tiles?

[msbarry]

MBtiles writing is a single-threaded bottleneck when generating output for the planet that takes up at least ~15 minutes mostly due to repeated ocean tiles:

It's possible to reduce mbtiles output phase to ~4 minutes (and reduce mbtiles size by 25%) by skipping filled tiles. Some clients can handle this by requesting parent tiles and overzooming when they get a 404. But some can't (or it might not be desirable) so this should be an opt-in feature if we want to do it at all.