Introduce maintenance_socket_authenticator and rework
maintenance_socket_role_manager to support role management operations.
Maintenance auth service uses allow_all_authenticator. To allow
role modification statements over the maintenance socket connections,
we need to treat the maintenance socket connections as superusers and
give them proper access rights.
Possible approaches are:
1. Modify allow_all_authenticator with conditional logic that
password_authenticator already does
2. Modify password_authenticator with conditional logic specific
for the maintenance socket connections
3. Extend password_authenticator, overriding the methods that differ
Option 3 is chosen: maintenance_socket_authenticator extends
password_authenticator with authentication disabled.
The maintenance_socket_role_manager is reworked to lazily create a
standard_role_manager once the node joins the cluster, delegating role
operations to it. In maintenance mode role operations remain disabled.
Refs SCYLLADB-409
