Feat: listen for unix signal

[dlhxzb]

When docker stop/restart, it would send SIGTERM and wait then SIGKILL.
Without listening for it we have to wait SIGTERM timeout here.
Fixes: #36

[rustdesk]

Please remove unnessessary format change, your format is different from ours (default VSC rust anaylizer).

[dlhxzb]

Is it possible that your VSC plug-in does not work at all 👀

use hbb_common::{
    log,
    rendezvous_proto::*,
    tokio::sync::{Mutex, RwLock},
    bytes::Bytes,  // No formatter would put `b***` here
    ResultType,
};

BTW either analyzer or cargo fmt, all used rustfmt as formatter

[rustdesk]

Sorry for this late PR merge, I put all my effort on rustdesk repo, and totally ignored this one.