Apache datasketches-postgresql-1.6.0

• Parallel query support (distributed aggregation)
• Added quantiles_double_sketch
• Added kll_double_sketch
• Fixed frequent_strings_sketch allocation
• Based on the latest datasketches-cpp 4.1.0 (upgraded from 3.1.0)
  • Substantial performance improvement of HLL union
  • Fixed a bug in Theta and Tuple union
  • Quantiles sketches default mode changed to inclusive
  • Fixed serialization of non-empty Theta and Tuple sketches with no entries
  • Support for older serialization versions of Theta sketch
  • Fixed serialization bug in KLL sketch