Because a `#connection_state` attribute will be added, I'd like to
consistently name the connection states everywhere they are used.

Tracking connection state can have many uses.  But in this version,
we're only tracking the session state and exposing a `#connection_state`
attribute reader.

For forward-compatibility, the only documented methods on the connection
state objects are (currently) `#name` and `#to_sym`, which will return
one of `not_authenticated`, `authenticated`, `selected`, or `logout`.

From RFC9051:
```
                   +----------------------+
                   |connection established|
                   +----------------------+
                              ||
                              \/
            +--------------------------------------+
            |          server greeting             |
            +--------------------------------------+
                      || (1)       || (2)        || (3)
                      \/           ||            ||
            +-----------------+    ||            ||
            |Not Authenticated|    ||            ||
            +-----------------+    ||            ||
             || (7)   || (4)       ||            ||
             ||       \/           \/            ||
             ||     +----------------+           ||
             ||     | Authenticated  |<=++       ||
             ||     +----------------+  ||       ||
             ||       || (7)   || (5)   || (6)   ||
             ||       ||       \/       ||       ||
             ||       ||    +--------+  ||       ||
             ||       ||    |Selected|==++       ||
             ||       ||    +--------+           ||
             ||       ||       || (7)            ||
             \/       \/       \/                \/
            +--------------------------------------+
            |               Logout                 |
            +--------------------------------------+
                              ||
                              \/
                +-------------------------------+
                |both sides close the connection|
                +-------------------------------+
```
> Legend for the above diagram:
>
> (1) connection without pre-authentication (OK greeting)
> (2) pre-authenticated connection (PREAUTH greeting)
> (3) rejected connection (BYE greeting)
> (4) successful LOGIN or AUTHENTICATE command
> (5) successful SELECT or EXAMINE command
> (6) CLOSE or UNSELECT command, unsolicited CLOSED response code, or
>     failed SELECT or EXAMINE command
> (7) LOGOUT command, server shutdown, or connection closed

Before the server greeting, the state is `not_authenticated`.
After the connection closes, the state remains `logout`.