Bug 1042939 - Resurrect periodic sync, alarms refactor r=millermedeiros

looking at my comment on the previous PR I think we still have the same "problem".. if you trigger multiple *syncs* or multiple *schedules* in a row it will keep adding more *actions* to the queue and won't really be smart about it. Check my implementation here: https://github.com/mozilla-b2g/gaia/pull/21827/files#r15716984