Firstly, this adds a missing assignment of `comm->proxy`, which would
cause the resolver not to know that a client asked via TCP/TLS, if the
proxy also asked via TCP/TLS.

Secondly, it merges `struct pl_proxyv2_sess_data` and
`struct pl_proxyv2_iter_data` into a single `struct pl_proxyv2_state`,
since they were practically the same. And it adds a bit of documentation
to the struct.