mbtiles: only create 1 unique index on tiles

[cldellow]

Before:

• the table definition created a unique index (visible in sqlite_master as sqlite_autoindex_tiles_1)
• we created an explicit unique index on closeForWriting

After:

• we create only the explicit index, at program startup

This reduces the mbtiles file by 0.5% and processing time by perhaps 0.3%, although it's quite noisy.

Creating the index at the start, rather than at the end, seemed to be fastest.

[cldellow]

Closing to re-open against master.