Before `sl:driver` was introduced, service levels were assigned as
follows:
1. New connections were processed in `main`.
2. After user authentication was completed, the connection's SL was
changed to the user's SL (or `sl:default` if the user had no SL).
This commit introduces `service_level_state` to `client_state` and
implements the following logic in `transport/server`:
1. If `sl:driver` is not present in the system (for example, it was
removed), service levels behave as described above.
2. If `sl:driver` is present, the flow is:
I. New connections use `sl:driver`.
II. After user authentication is completed, the connection's SL is
changed to the user's SL (or `sl:default`).
III. If a REGISTER (to events) request is handled, the client is
processing the control connection. We mark the client_state
to permanently use `sl:driver`.
The aforementioned state `2.III` is represented by
`_control_connection` flag in `client_state`.
Fixes: scylladb/scylladb#24411
c02535635e