Make the Downstairs own per-connection state (#1328)

This PR moves ownership of the `ConnectionState` from a local variable
in `proc_task` into the `struct Downstairs`.

It is preparation for consolidating Downstairs logic into a single task,
instead of the current task-per-connection architecture (in which
multiple tasks share an `Arc<Mutex<Downstairs>>`).

In the `Downstairs`, per-connection state is indexed by a new
`ConnectionId` type, which is a monotonically increasing counter that
increments on every new connection. We're using a new type because
upstairs / session UUIDs are sometimes reused if a new connection is
made, and we specifically _don't_ want to reuse `ConnectionState`
between different connections to an upstairs with a maybe-matching UUID.
In the previous code, this was ensured by using local variables in
`proc_task` (which was spawned on a per-connection basis); now, we
ensure it by using `ConnectionId` (which is also unique on a
per-connection basis).

This reorganization had a few other side effects:

- Downstairs replacement is extracted from `proc_task` into
`Downstairs::on_new_connection_replacing` (with minimal logical changes)
- Message handling is extracted from `proc_task` into
`Downstairs::on_message_for` (with minimal logical changes)
- `proc_frame` now takes a `&mut Downstairs` (instead of a
`Arc<Mutex<Downstairs>>`), because it is called from `on_message_for`
(which already has the lock)

Author: mkeeter