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>