Krikri::LocalPluginDaemon::LocalDaemonKey

(inventory host name, become_user-or-nil). The host_name dimension keeps the one-fiber-per-host promise: two local-connection hosts are two fibers, so they get two daemons and never share a pipe. A nil become_user is the unprivileged daemon; each distinct become_user gets its own, exactly like SSHManager's table.