parallel Import-sql, speedy Postserve, performance testing

Many improvements with this release, mostly around MVT, SQL importing, and Performance testing

• The Postserve realtime tile server is now much more robust, supporting multiple parallel queries and request cancelations [@eyusupov, @nyurik]
• Improvements to SQL importing [@nyurik]
  • The import-sql has been merged into this repo.
  • generate-sql can now generate multiple .sql files that can be imported in parallel
  • import_sql.sh can import multiple sql files in parallel
• New test-perf tool to evaluate tile generation performance against a PostgreSQL database. [@nyurik]

This utility can test individual layers, several layers at once, as well as the whole tile. It has several pre-defined testing areas to provide cross-test consistency. Results are printed as histogram graphs, showing tile size distribution. The utility can compare current run with a previously saved one, highlighting large changes. If multiple zoom levels are tested, another histogram shows per-zoom size distribution.

• Expanded MVT generation utility with numerous new features [@nyurik]
  • support for server side gzipping,
• Support for the standard PostgreSQL environment variable naming like PGHOST, PGUSER, ... in addition to the legacy custom POSTGRES_HOST, ... [@nyurik]
• Bug fix for the mapping diagram issue, not generating images for some layers. [@nyurik]