Report server proxy epoch to broker

[doyoubi]

The metadata storage needs to know the real epoch for server proxy for the following reasons:

• For scaling down, the broker needs to know when the latest metadata have already been synchronized to all the related server proxies to safely remove server proxies out of the cluster.
• The metadata storage needs to know all the up-to-date server proxies to provide an API to query all the ready-to-use server proxy endpoints.
• The metadata storage can use real epochs to detect inconsistent metadata.