move epoch off of client object and to socket object

[sjpotter]

Description

I think keeping track of the "epoch" makes more sense on the socket than on the client itself, as its tracking connections, and that's a socket issue, not a client issue.

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Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?