EPP protocol: proxy<->epp remove the buffered code path

[ahg-g]

What would you like to be added:

We recently added support for full-duplex mode: the proxy can stream the request/response bodies to the EPP. I would like to make that as the one and only mode of communication between the epp and the proxy.

Why is this needed:

To simplify the protocol, the code and the deployment of EPP.

[danehans]

+1

[ahg-g]

We made streaming the default in #552, next we should remove the buffered code path.

[ahg-g]

/assign @kfswain

[kfswain]

Fixed in #672