Server.wait(for:)

Runs in a process of its own even on a connected server, because a control client runs one command at a time: carried over the connection this would hold back every call beside it, Server/signal(_:) included, and nothing would ever release it.

• Throws: TmuxError/serverRestarted if the server went away while this was waiting. tmux releases its waiters when it shuts down, with the same silent success a real signal produces, so the server's identity before and after is what tells a release from a departure. TmuxError/cancelled if the task was cancelled, which ends the wait without a signal.