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>
Define NeoFS location database based on BoltDB. Implement methods to save
the record by key (Put) and to read the record by key (Get).
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
