Support for legacy postgis 2.4dev image, MVT improvements

BREAKING

• Removed --no-tile-envelope parameter from generate-sqltomvt, postserve, test-perf.
  Now ST_TileEnvelope() support is automatically determined by the PG version - @nyurik

Improvements

• Added MVT support for the legacy Postgis 2.4dev (current docker image) - @nyurik
• MVT tools now customize generated SQL based on the PostGIS version - @nyurik
• Make EXPIRETILES_ZOOM customizable in import-osm - @francois2metz
• debug-mvt now prints total MVT layer size, and can print <null> in values - @nyurik