Support changing manual configuration while running

[ianpaul10]

Ref #380

• Load config at the start of each find task
• Only load config and return the new values if the file's getmtime has been updated, otherwise return the cached peers
• If anything fails when trying to read the config file, return the cached peers (or empty dict if it's never been loaded)
• Add new cleanup peers task that removes peers from the known_peers list if they no-longer exist in the dict but still exist in the known_peers list
• Test that starts with 2 peers in the config file, adds a 3rd to the config, checks that it gets picked up by a node, and then remove it from the config, and check that it gets forgotten by the node.

[ianpaul10]

@AlexCheema This PR should address the follow-up to updating the manual config while exo is running. Please take a look when you get a chance!

[ianpaul10]

@AlexCheema Thanks for the notes, reading from the config file should now be non-blocking

[ianpaul10]

@AlexCheema Bump. I think this should be ready to merge now that the lock is removed and simplified.

[AlexCheema]

Merged! Certainly worthy of a $200 retrospective bounty. Please email [email protected] to claim the bounty!