Add support for server lifetime

[chipsenkbeil]

Terminates client/server after some period of time, regardless of active connections. Designed for security at work. Default should be 8 hours.

[chipsenkbeil]

Used to have this as --shutdown-after. The annoying thing was that this was done by checking if we had no active connections to the server. Instead, we probably want multiple options:

1. Never expire
2. Expire after X amount of time regardless of connections
3. Expire after X amount of time with no connections

[chipsenkbeil]

Could rename --shutdown-after to --shutdown and provide an enum that parses a string.

1. --shutdown never will yield Shutdown::Never
2. --shutdown [lonely=]<number> will yield Shutdown::Lonely(Duration) (compatible with old --shutdown-after <number>
3. --shutdown always=<number> will yield Shutdown::Always(Duration) that will shut the server down after N seconds regardless

[chipsenkbeil]

Resolved by https://github.com/chipsenkbeil/distant/commit/74a37209eb2e973200011ed19d86ee8840cea1f4.

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