sync: Keep a moving average of last fetch times

Try to more accurately estimate which projects take the longest to
sync by keeping an exponentially weighted moving average (a=0.5) of
fetch times, rather than just recording the last observation. This
should discount individual outliers (e.g. an unusually large project
update) and hopefully allow truly slow repos to bubble to the top.

Change-Id: I72b2508cb1266e8a19cf15b616d8a7fc08098cb3
1 file changed