Reorg endpoint, fix flaky test, fix Host Ipv6 parse and display

[TheButlah]

I'm trying to break up #72 to be a bit smaller, so I've manually extracted some of the changes from there to this separate PR.

This PR reorganizes the endpoint module to break it up into a few submodules. This helps readability and makes it easier to navigate that code.

I've also fixed some issues with how endpoints and hosts displayed and parsed IPV6 addresses. I've reworked the semantics of Host::Ipv6 to more closely match the semantics of std::net::Ipv6Addr, and added associated tests. This removes the need for the format!("{}:{}", host, port) that I was doing whenever I passed a host and port tuple to tokio, and correspondingly fixes the bugs in the parts of the code that weren't using that.

I also fixed a flaky test - as mentioned in #73, our sockets are not yet resistant to ordering issues when connecting to an endpoint that might not yet be bound. For this reason, we now await until the pub socket has finished binding in the pub_sub test