Redis-based authentication lacks user synchronisation

[retrixe]

This has become a problem since c665136

We could handle this intelligently:

• Main Octyne instances will have accounts in their users.json which are loaded to Redis.
• Subordinate Octyne instances will not have any accounts, and will only read accounts from Redis.
  These instances will disable their account management endpoints.
• We could add a role property to the Redis configuration to prevent misconfiguration.

Loading accounts could be done via the same Authenticator interface i.e. RegisterAccount and UnregisterAllAccounts functions.