feat: gossip config reload information

[kentquirk]

Which problem is this PR solving?

• When a configuration is reloaded (and it has changed from the previous version) this publishes the news of the change by publishing the configuration hashes. Other peers will receive this message and immediately reload their configs.

Short description of the changes

• Because config needs to be loaded to start the app, it can't depend on pubsub which hasn't been set up yet
• So I created configwatcher, which lives outside config and can have both config and pubsub injected
• It registers with config for callbacks, AND with pubsub for messages from peers
• It publishes news of config changes with a timestamp
• When it receives a message it checks the hashes; if they're different, it reloads
• It publishes news of config changes only if it hasn't received a message in a little while (controlled by config)
• Config tests had to be moved to config_test package in order to test reloading in this new world