The prior implementation involved some (moderately awkward) layered
`match` expressions; this version skips those in favor of `?`. The cost
is a little less detail about (for example) when serialization errors
happen. On the bright side, it gains us some huge improvements in code
clarity. We might be able to get back the tracing later, with other
library support.
In addition, we know have a couple tests: one to make sure we choose the
authentication method appropriately, and a very basic handshake test for
when we're not actually negotiation a username and password.
I believe these were introduced previously to solve a problem that we're
no longer dealing with; specifically, if I remember correctly, we
introduced these to deal with how we were going to implement a trait.
However, they don't appear to be necessary any more, so we're going to
get rid of them, so we won't need to deal with them any longer.
