DRILL-3496: Augment logging in DrillConfig and classpath scanning.
Main: Added and augmented logging calls.
Code Hygiene:
- Added/revised doc. comments, code comments.
- Renamed some locals.
- Added "final".
- Wrapped lines.
Also fixed currentTimeMillis -> Stopwatch in Drillbit.
Conflicts:
common/src/main/java/org/apache/drill/common/config/DrillConfig.java
