Generate tiles by PGquery, layer-stats tool, generate bbox

Main features

1. A script to generate vector tiles from PostGIS' ST_AsMVT using tilelive-pgquery (thanks @nyurik) - Thanks @lazaa32 for implement #317 and @smellman, @zstadler and @nyurik for the original script
2. Remove download-osm make-dc, create a simple text file with bbox value instead (#297) - thanks @nyurik
3. New layer-stats tool to explore data (#293) - Thanks @nyurik for implement based on OpenMapTiles / qa

$ docker-compose run --rm openmaptiles-tools layer-stats frequency openmaptiles.yaml aeroway class
======= Analyzing value occurrence in layer 'aeroway' for field [class] =======
class        z10    z11    z12    z13    z14
---------  -----  -----  -----  -----  -----
aerodrome    151    151    154    154    154
apron         56     56    126    224    308
runway       470    470    515    525    528
taxiway     1075   1075   1156   1219   1245
heliport                     1      2      2
helipad                            11    162

Improvements

• Improve delete_empty_keys() performance, add ps to tools (#295) - thanks @nyurik
• Remove the double set of the header Access-Control-Allow-Origin. (#296) - thanks @francois2metz
• preserve mapping order (#302) - thanks @remi-dupre
• Use print_err in places that mix errors and output on stdout. (#307) - thanks @lseelenbinder
• layers without attributes in yaml mapping (#319) - thanks @frodrigo
• Stop parallel import-sql on error (#322) - thanks @frodrigo
• Use pg_roles view instead of pg_authid table (#323) - thanks @baipi

Fixes

• mbtiles-tools fix broken --show-json param (#304) - thanks @nyurik)
• Fixed render scheme in generate-vectortiles (#305) - thanks @kukulich

Clean ups

• #294 (thanks @nyurik), #299 (thanks @erictheise), #301 (thanks @mattcen)