MBTilesTileStore deduplication

[fredj]

Using view, see https://github.com/mapbox/node-mbtiles/blob/master/bin/mbcompact

[elemoine]

I'm curious to understand what this issue means.

[fredj]

The idea is be able to "compress" a mbtile file by not storing the same tile data again and again.

Instead of having a tiles table, we have a map table that stores the tile coordinates and the md5 checksum of the data and an images table that stores the same checksum and the actual data.
The tiles is replaced by a view.

When inserting a new tile, md5(tile.data) is computed and the data is inserted into images only if not already present.