Re-use the regular logger for performance data
As per !98 (diffs, comment 16839564) using a separate logger in the current setup won't work. Re-using the same logger as used for regular log messages also means we don't need to set up any extra log rotation and what not. This does mean we'll need to prefix performance log lines with something, e.g. "metrics: command-add-keys cpu_time=XXX wall_time=XXX". This would produce log messages such as:
I, [2016-10-12T15:53:37.714193 #17935] INFO -- : metrics: command-add-keys cpu_time=1.2 wall_time=2.0