Generate tile size statistics

[systemed]

(A little bit niche but...)

It would be useful to optionally generate aggregate output tile size statistics. For example, total tile size for each z8 tile (and the z9, z10... tiles underneath it).

In theory one can do this after the event in SQLite by summing the LENGTH of each blob, but IME this is desperately slow whereas it would take tilemaker fractions of a second.