idea: follower brokers actively track lead broker configuration

[garlick]

Problem: currently we assume the same TOML configuration files are synced across the cluster, but in practice sometimes this is inconvenient.

While configuration that is used early such as [bootstrap] and [tbon] and the CURVE key must be read from disk on all ranks, we could have each broker make a streaming request to its parent to fetch the current config and any updates, and then ignore on disk changes everywhere except rank 0. Module loading could be delayed until the first response is received.

That might make configuration easier to manage.