[Feature] Support more message types in server

[DavdGao]

Our Target

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

What's in this PR

• This PR is to support more message types in the server (currently we only support train and eval in self.msgbuffer)
• We add a enumeration class STAGE in enums.py to avoid using string directly
• We modify check_and_move_on and check_buffer fucntions to support more messages.
• The modification is compatible with previous versions

[rayrayraykk]

I'm not sure whether the 'stage' is an appropriate key name, as the key of msg_buffer should be message type (although it is not now: msg_buffer['train'] contains model, msg_buffer['eval'] contains eval results).

Maybe we should use the message type as the key of msg_buffer (like msg_buffer['model_param'] ).

What's more, are STAGE and state confusing?

[rayrayraykk]

Please see the inline comments, and the other part looks good to me.