Freezes now show up in the log, with their cause

Obeya sometimes froze for a few seconds, for example right after a card was approved, and nothing said why. Now every freeze of half a second or more is logged: how long the server stood still, where in the code the time went, and which git commands it waited for. A browser page that freezes reports this too, so the same log names the page and, in Chrome, the script that held it up. The entries also go into stalls.log in Obeya's home, so freezes can be tracked down systematically instead of guessed at.