Agent protocol version

Two are the highlights of this:

* body server control allowlegacyconnects ACL
* body common control protocol_version

Necessary for the common control protocol_version was to change the libcfnet API. So the old API that was receiving FileCopy attributes is gone. Now there is a new ```ConnectionFlags``` type that is passed to ```ServerConnection()```. Handling of connection caching is done by manual looping outside libcfnet and inside cf-agent, since it's the only user of our bogus connection cache.