Add multi-node integration tests

[ethanfrey]

This is a series of slower tests in a more realistic environment that can be run occasionally (eg. only on master merges, not by default in yarn test).

Somehow setup 4 node testnets with docker (compose?) and connect them. Use staking to create wildly changing validator sets, and add/remove/stop a node. Ensure we can update light clients even when the validator set changes quickly and handle "real world" issues