Changes:
* replace `iotuil` elements with the ones from `os` package;
* replace `os.Filemode` with `fs.FileMode`;
* use `signal.NotifyContext` instead of `NewGracefulContext` (removed).
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Scanning subdivision csv-table entries one-by-one takes significant time and
system resources. To speed up random access to table records, on the first
call, the table is pumped into memory (map). On subsequent calls, I/O
operations are not performed.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
