Refactor sampled test queries into a separate abstract class
Distribute splits evenly among nodes
Experimental support for approximate queries

Currently only AVG is supported. The syntax is:
SELECT AVG(a)
FROM my_sampled_table
APPROXIMATE AT 95.0 CONFIDENCE

This is disabled by default, but can be enabled with the config option analyzer.approximate-queries-enabled=true
Ensure all page buffer clients are closed
Update to latest airbase, airlift and discovery
Move TestingPrestoServer from test to main
Make primary JDBC artifact standalone
Fix LocalExecutionPlanner handling of null literal projections

Disable compiler tests involving untyped nulls and mixed numeric types