Sync scheduling: Only retry up to twice after fail. #2386

Previously when a sync failed, we'd retry very soon (30s) no matter how
often a sync had failed before. After this change we'll retry twice and
then back off to the regular 5min interval.
Sync scheduling: Only do a follow-up sync 3 times. #2355

After trying again immediately for 3 times, no more fast follow-ups
are run until a sync finishes without needing a fast follow-up.