Support importing multiple mbtiles files that update the same map

[achou11]

Running into a situation where there's a large mbtiles file that is hard to import on a mobile device due to its size. in theory, one could split the file into a few smaller files and import them separately to create the same resulting map. unfortunately, the current implementation does not support this because each time we extract the metadata and convert to a tilejson format, we create a new id, so each import would create a new map instead of updating the same one.

Proposed solution is to update the id generation in mbTilesToTileJSON to support a user adding a custom MAPEO_ID metadata field to the mbtiles metadata table, which could then be used to determine the id i.e.

id: metadata.MAPEO_ID || generateId(),