support multiple process use same log file

[chenfengyuan]

let mut config = fast_log::Config::new()
        .custom(ConsoleErrAppender {})
        .file_split(
            file_path,
            fast_log::consts::LogSize::MB(log_file_mb),
            fast_log::plugin::file_split::RollingType::KeepNum(log_file_keep_num.into()),
            LogPacker {},
        )
        .level(log::LevelFilter::Info);
    config.format = Box::new(MyLogFormat {});
    fast_log::init(config).expect("failed to init log");

Currently fast_log will open file with read + write, and seek to the file end, and write logs to file.
When multiple process use same log file, it will result in repeated overwriting same position.
Opening the file using append can solve this problem.
But additional code may be needed to determine when to roll the file and synchronize this state between multiple processes.