Landcover related landuse should all show at zoom 9

[nvkelso]

http://localhost:8007/#9/38.1648/-121.2671

We're showing some landcover type landuse starting at zoom 9 (like grassland), and others at zoom 10 and other zooms. They should all come in at the same zoom (9), when we switch to OSM data source.

Versus zoom 10, which adds farmland:

[nvkelso]

tl;dr

• Looks like farmland is the big offender here. It needs to start at zoom 9 not 10.
• We may also want to move orchard to zoom 9 as well.
• Generally we include the rest at zoom 9 already if they are large, and throttle the smaller ones for later zooms (eg filter: { landuse: forest } with min_zoom: { max: [ 9, *tier2_min_zoom ] }. That seems to be working for us, but it could also be creating holes for us.
  • Around Concord, California (11/37.9512/-121.9860) I zoom in / out and I see little polygons being dropped at lower zooms

Research:

Related to #1259 and #1788.

From that issue's description of low zoom landcover:

landuse IN
  farmland (and farm), modify the min to be 9 instead of 10.
  orchard, starts at 10 now, should be 9?
  forest – do we need a custom `tier2_min_zoom` to show all at zoom 9?
  residential - okay to start at 10, because of NE overlap
  commercial - okay to start at 10, because of NE overlap
  retail - okay to start at 10, because of NE overlap
  industrial - okay to start at 10, because of NE overlap
  meadow, starts at 9, but throttled
  vineyard, starts at 9, but throttled
natural IN
  wood – do we need a custom `tier2_min_zoom` to show all at zoom 9?
  sand - starts at 9, but throttled
  scree - starts at 9, but throttled
  shingle - starts at 9, but throttled
  bare_rock - starts at 9, but throttled
  heath - starts at 9, but throttled
  grassland - starts at 9, but throttled
  scrub - starts at 9, but throttled
natural IN
  wetland - starts at 9, but throttled
  mud - starts at 9, but throttled

[nvkelso]

I also wonder if we should be including all the "urban" content in a big union separate from any remapping – that way we'd get maximum urban_area polygon at mid-zooms, while preserving when individual overlays like hospital, school, parking lot come in?

See 12/37.9778/-122.0918.

[nvkelso]

Most this pink forest is only visible at zoom 10 not zoom 9 around 10/32.8367/35.6391 in Israel.

[nvkelso]

Around Watsonville, California (12/36.8614/-121.7199)

[zerebubuth]

Related: #1794.

[nvkelso]

Moving the patchy landuse subissue here to it's own issue for next milestone in #1822.

[nvkelso]

The final QA changes for min_zoom are verified. Here for farmland: