design the distributed version of the DAG store for 1:N miner/markets

Potential design direction: every markets node runs the dagstore under strong consensus (e.g. Raft/Paxos style) with all other markets nodes. Potentially with a shared transients/index location, or with a protocol to transfer those. In other words, we don't foresee an active/passive setup, but rather an active/active one where all nodes run fully capable dagstores.