DRILL-3216: Part 1--Pre-core code hygiene.
Many-instance changes: [DatabaseMetaDataGetColumnsTest]
- Renamed "...opt..." columns and methods to match "...mdrOpt" variables.
- Renamed "...req..." columns and methods to match "...mdrReq" variables.
- Renamed "testRow..." variables to "mdrUnk".
- Added/applied getIntOrNull(...) especially to simplify null cases.
- Put REAL, FLOAT, and DOUBLE in that order (smallest, ~variable, largest).
- (Edited "nullability code" -> "nullability code:".)
Renamed "type" -> "relDataType"; "sqlType" -> "sqlTypeName".
Changed "dfs_test.tmp" -> VIEW_SCHEMA.
Re-aligned query SQL.
Added 2 "final".
Edited 3253 (test plugin) TODOs.
Purged several comments.
Wrapped some lines.
