DRILL-3159: Part 1--Prep., Hyg. for: Make JDBC throttling threshold configurable.
Cleaned, enhanced DrillResultSet:
- Enhanced ResultsListener logging:
- Added instance ID; added batch numbers.
- Added logging at close (pairing with logging at construction).
- Fixed 2-integer query ID to UUID form.
- Renamed qrb -> qdb; q -> qdb (per recent QueryDataBatch change).
- Added "final" on ResultsListener's logger.
Reduced Avatica-vs.-Drill casting:
- DrillStatementImpl's (Drill)Connection(Impl).
- DrillResultSetImpl's (Drill)Statement(Impl).
Converted a comment in ExecConstants.
