Draft: Refactoring Agent

Hi,

as it currently stands, Agent is structured quite weirdly (for understandable historical reasons)

For instance, Agent::send_message calls out to message::send::send_message, passing out a reference to itself, which then calls back to Agent::client on which it calls send_stanza.

The purpose of this MR is to try to disentangle and refactor Agent to remove this awkward structure and improve maintainability. It's mostly a bundle of small changes that wouldn't entirely make sense to make a MR out of on their own.