Throttle CPU usage properly when cloning

[rpowis]

Problem

We can be a bit smarter about how we're throttling the CPU when cloning the repos. A quick fix was put in by #31 but it's using a number that doesn't have clear meaning as to why it was chosen.

Ideas

• Track the upper/avg for the amount of memory each clone uses, before kicking off a new clone look at freemem to see if there's enough to perform another clone, if not poll freemem until we have enough