Writing transactions

[zverok]

An idea for far future, extracted from discussion:

...support for some sort of "transactions" for writing could be useful, like

• transaction writing to that db: df1 to table1, df2 to table2, then commit;
• transaction writing to that CSV file: df1, then df2, then flush;
• transaction writing to that XLSX file: df1 to sheet1, df2 to sheet2, then save.