DRY connection/channel updates

[colin-axner]

Now that chain.UpdateClient does all updating functionality. I'm pretty sure we can reduce several lines of code by calling src.UpdateClient and dst.UpdateClient in the beginning of connection/channel handshakes and using those messages when necessary. To fix this issue, simply construct the update message for src and dst once, at the top of ExecuteConnection(Channel)Handshake, and pass those into Initialize, and use the constructed messages instead of creating one in each switch statement