Tweak spawn token defaults and add logging

Follow-up to #1258 (comment 84839403)

I want to have a way to detect if a Gitaly server is slowing down. Maybe the spawn token aquisition time will let us do that. Right now the limits are so permissive that we only see metrics / log data when things are really really bad. By adding more logging we can see if this is also a good indicator of trouble during a slow-down.

My hypothesis is that the log messages created by this MR will peak during a slow-down. If so we can use this as a signal to shed load and free up unicorns.