Implement GroupManager #550

GroupManager is a new abstraction which will keep track of provider groups when sending bids on bidder nodes. By default we will have one group which consists of all providers with > min_stake. This will change if provider stake goes below minimum or if bidders configure a provider-whitelist to restrict providers.

The GroupManager will consume events provided by the new StakeManager (#549) and reconfigure the groups if required using the KeyExchange protocol.

The GroupManager should be smart enough to figure out if peer disconnection should result in new encryption keys being configured. Also be able to react to future events or multiple L1 chain groups in future.