Batch repository synchronization drush command

The repo sync drush command (vcapi-parse-logs) is a potentially VERY long-running command. On util, it ran for 18 hours before exhausting 840M of memory and only getting through ~25% of all the d.o repositories. It desperately needs batching, as that'll hopefully reduce the amount of bloat and slowdown that occurs, as well as increase the speed through some concurrency (I hope, maybe?).

This is a good thing to do in general, though if we don't get the concurrency out of batched drush, then it's less useful. Still worth doing, but not as useful.