Support for server with single "main" node and several proxies

When serving many clients, it might be beneficial to do so from several OS processes, rather than a single large one. We could do this by having a single main node, and then several proxy nodes that follow the main node using a (local) chain sync protocol. ideally, the proxies and the main node should then share (almost?) all of the on-disk storage, just keeping their own in-memory data structures.