High CPU usage on steps with high freq output

[preed]

CPU usage on certain steps has gone way up since the logging code landed (see commit d84ec6f); this is particularly noticeable in the FirefoxExtractSource step of FireboxGetSource process example.

In normal builds, we're seeing reasonable CPU usage, so it might just be that certain tools that do a lot of output (like tar) are causing this problem; we should see if we can either put in some caches to reduce the number of log messages that get sent around the system, or somehow otherwise reduce CPU usage.