db_xml.py: cleanup_table(): Inline sql.pkey_col ('row_num') because this is the only place it's used

cleanup_table(): Use new sql.table_cols() instead of sql.table_col_names()

sql.py: Added table_cols()

db_xml.py: put(): Fixed bug where needed to avoid truncating the pkeys_loc table, in case it's the same as one of the in_tables. This occurs now that sql_io.put_table() passes through the actual input column instead of the joined-together input table's column when ignoring all rows.

sql_io.py: put_table(): Resolving default value column: If ignoring all rows, use input cols directly instead of cols from joined-together input table. In addition to being simpler, this prevents the returned column's name from growing longer and longer as each iteration prepends its input table table name to the default value column name.

sql_io.py: put_table(): Moved changing the table of the default value column from Resolving the default value column to Setting pkeys of missing rows, because the table change is only needed in this section

sql_io.py: put_table(): Resolving default value column: Always call sql_gen.remove_col_rename() because it will just pass the value through if it's not a column

sql_gen.py: simplify_parens(): Removed extra simplify_parens() at end because it is done in the final iteration that performs no other replacements, so it is not necessary to also do it explicitly

sql_io.py: put_table(): Replaced limit_ref integer with ignore_all_ref boolean, because it is no longer used as a select statement limit

sql_io.py: put_table(): remove_all_rows(): Corrected "just create an empty pkeys table" comment to "just return the default value column"