This commit adds the infrastructure needed to test per user sla,
more specificaly, a service level accessor that triggers the
update_service_levels_from_distributed_data function uppon any
change to the dystributed sla data.
A test was added that indirectly consumes this infrastructure by
changing the distributed service level data with cql queries.
Message-Id: <23b2211e409446c4f4e3e57b00f78d9ff75fc978.1609249294.git.sarna@scylladb.com>