[WIP] Add client manager in server

[DavdGao]

Target

• We are planning to add a consultation stage before training for differential privacy and other algorithms.
• We decompose it into several PRs to avoid large modification at one time.

What's in this PR

• This PR is to add client manager in server, which is responsible for
  • Maintain the status of clients, e.g., working, idle or unseen (in training)
  • Manage the clients, including registration, client-related information storage (client_resource, [TODO]join_in_info)

Why

• Currently, the client management is too decentralized. For example,
  • self.join_in_info stores the client information,
  • self.id_unseen_client records the unseen clients during training and
  • self.sampler records the status of the clients (Maybe sampler should be in charge of the client status)