DRILL-3285: Part 2--Renaming.
Renamed state/control-flow members:
- started -> initialSchemaLoaded
- first -> beforeFirstBatch
- redoFirstNext -> returnTrueForNextCallToNext
- finished -> afterLastRow
Renamed other items:
- changed -> schemaChanged
- currentBatch -> currentBatchHolder
- DrillResultSet's currentBatch -> batchLoader
