[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