Move a few methods into downstairs (#1160)

At some point, we may want the `Downstairs` to own its data.  This PR moves
`proc_frame` and the core of `do_work_task` into methods on the `Downstairs`,
though they still take a `Mutex<Downstairs>` as their argument.

The purpose of this PR is to make later PRs more readable: this is mostly
mechanical moving + indenting of code, so that a later PR that changes logic
won't also have 500 LOC of reorganization.

I did sneak in _one_ small change, which is
[consolidating calls to `is_message_valid`](https://github.com/mkeeter/crucible/blob/29185f5538632a13039785e911d1aaa0146bfa14/downstairs/src/lib.rs#L2254-L2312)

Author: mkeeter