WIP: aggregator should be able to lead without data

[andrewmains12]

WIP: please don't review just yet :). I'll add full context here once this is in a more stable state.

tl;dr: Without data flowing through the system, the aggregator follower never reaches a point where it knows it can take over a shardset (CanLead = true). We can fix this by persisting a heartbeat to etcd before there's data.