Looking back multiple zooms for fill tile

[dnomadb]

The fillzoom capability of tilelive-vector works well for a tileset with one consistent effective maxzoom, where a fillzoom tile should exist for all child tiles at higher zooms. However, for a "bumpy maxzoom topography" -- where the effective maxzoom of existing tiles is spatially variable -- the ability to load the highest existing parent zoom tile (within a reasonable threshold) would be useful.

A lookback, when specified in the info for a given tileset, would describe the number of zooms that should be looked for, and the highest zoom of these used to overzoom and fill in the missing tile.
For example, when trying to load a z19 tile, where a z17 tile exists, and the lookback is 3:

1. Z19 tile attempts to load, but is not found
2. Z18, Z17, Z16 tiles all all attempted to load
3. The highest zoom found is overzoomed

cc @yhahn

[dnomadb]

Working on this here: #127