This PR brings few modification which make it possible to generalize landcover on larger areas. Tested on Europe.
- Use ST_SnapToGrid to avoid ERROR: GEOSUnaryUnion: TopologyException: found non-noded intersection between LINESTRING.
- Cluster only landcover subclasses: wood and forest.
- Use less generalized tables as a starting point for next step generalization.
Instead of using [Douglas-Peucker algorithm](https://postgis.net/docs/ST_SimplifyPreserveTopology.html) which is using for generalized tables [imposm](https://imposm.org/docs/imposm3/latest/mapping.html#generalized-tables) it is used [Visvalingam-Whyatt algorithm](https://postgis.net/docs/ST_SimplifyVW.html).
Solution:
remove imposm generalized tables (during `import-osm`) and create generalization in `import-sql` step for zooms 7-14. Zooms 0-6 are from Natural Earth data.
Upper zoom levels (7 and 8) are ok to merge with no big impact on creation speed. In Canada from z9 it took too long. Between zoom levels 10 - 13 there can be union polygons with less than 300 edge points (empirical number based on a test on Canada forest, can be discussed). Polygons with more than 300 edge points are just simplified. Zoom 14 is from the original dataset `osm_landcover_polygon`.
There is also removed `osm_id` which is never used and not passed into vector tiles.
