Fix spawning of terminfo thread in server mode

[lihaoyi]

Previously we only spawned the terminfo updater when spawning the Mill server process, but that only handles the no-server case and first-time-spawning-server case, without handling the subsequently-re-use-server case.

This moves the logic into a helper method so we can call it from both server and no-server launchers at the first point at which we have a dedicated serverDir available. Since the serverDIr is only available deep inside the ServerLauncher object, as it needs to do the work of testing different locks to pick a server, we add a preRun hook that we can override to start the terminfo updater thread