Removed no longer used schemas/vegbien_empty.sql

main Makefile: Removed empty_db, because `make schemas/reinstall` has the same effect and is simpler

README.TXT: Changed documentation to use make schemas/reinstall to empty the DB, since that command is simpler. Added how to archive the last import.

db_xml.py: put_table(): Removed `if not db.debug_temp` check because that is done by sql.empty_temp()

sql.py: put_table(): Use new empty_temp()

import.stats.xls: Added comments for estimated numbers. Added "," separators to large numbers.

sql.py: empty_temp(): In debug_temp mode, leave temp tables there for debugging

sql.py: empty_temp(): Don't output at log_level 2 because it's an internal query, not part of the core algorithm

sql.py: truncate(): Added kw_args to pass to run_query()

Added inputs/import.stats.xls, which compares row-based and column-based import. This shows that column-based import is slowed down by table locking when run simultaneously, so we will need a new INSERT IGNORE replacement that doesn't lock tables.