sync: Order projects according to last fetch time

Some projects may consistently take longer to fetch than others, for
example a more active project may have many more Gerrit changes than a
less active project, which take longer to transfer. Use a simple
heuristic based on the last fetch time to fetch slower projects first,
so we do not tend to spend the end of the sync fetching a small number
of outliers.

This algorithm is probably not optimal, and due to inter-run latency
variance and Python thread scheduling, we may not even have good
estimates of a project sync time.

Change-Id: I9a463f214b3ed742e4d807c42925b62cb8b1745b
1 file changed