print "still waiting" for long running commands if we serialize on them

Heuristic: if the currently-running count drops to 1, and that process has been running for 3s+, then print-and-shame.

It won't work if soon-to-be-long-running process just started with at least one other process, and then before 3s has elapsed, all the other processes terminate.

Similar to https://github.com/martine/ninja/issues/111 but I didn't see a simple way of doing exactly that (because if nothing's happening I think we're blocked waiting for processes?). I might be misreading though.