Feature Request: --one-tile option for tile-join

Nobody has claimed this yet.

I have a similar situation to https://github.com/mapbox/tippecanoe/issues/109 where I am constantly amending an mbtiles with new geojson using tile-join. I have a serverless backend that serves vector tiles. Sometimes the mbtiles requires many updates in quick succession so re-generating, deleting and uploading all the files for the standard output wasn't feasible. Currently I fetch the mbtiles, generate the tiles and return the requested x/y/z. This works but it still requires me to generate hundreds of files on each request (they are cached but still), it would be great if tile-join had a `--one-tile` property similar to tippecanoe.

To summarize, for updating tiles I do essentially:
- fetch `existing.mbtiles`
- `tile-join -f -o /tmp/existing.mbtiles /tmp/new.mbtiles /tmp/old.mbtiles`
- `upload new.mbtiles` as `existing.mbtiles`

Then on each vector tile request to /endpoint/z/x/y
- fetch `existing.mbtiles`
- `tile-join -f -e /tmp/tiles -z18 --layer=run /tmp/existing.mbtiles` <- this generates the entire structure unnecessarily
- return `/tmp/tiles/z/x/y.pbf`

Admittedly I'm not sure how much of a performance gain I'd get by only generating one tile since I don't know the implementation details but this would be helpful. If you know of a better way of doing what I'm doing that I'm open to that as well, thank you.

Start by reviewing tile-join's handling of the -e/--output-to-directory and -z options, then compare the related --one-tile behavior in tippecanoe referenced by the issue. Define how tile-join should receive the requested z/x/y and verify that only that vector tile is produced while preserving the existing MBTiles workflow.

Written by the indexing model from the issue text.