standardize table storage and scans

[aheev]

• add createScanState
• get rid of arrowTable::getNumBatches
• rm arrow/icebug-disk dynamic_cast checks wherever possible

[aheev]

@adsharma could you PTAL?

[aheev]

failing test is unrelated:

EXPECT OK BUT GOT ERROR: IO exception: Failed to create directory /home/runner/.lbdb/extension/0.17.0/linux_amd64/ due to: IO exception: Directory /home/runner/.lbdb/extension/0.17.0/linux_amd64 cannot be created. Check if it exists and remove it.

[aheev]

This refactor has been carried out based on the assumption that any new external table formats will strictly conform to ColumnarTableBase. Otherwise common functions like getNumScanMorsels and createScanState would need to be moved to NodeTable / RelTable classes