Major compaction can be started from both storage_service and column_family
api. The first allows to compact a subset of tables in given keyspace,
while the latter - given table in given keyspace.
As major compaction started from storage_service has a wider scope,
we use its mechanisms for column_family's one. That makes it more consistent
and reduces number of classes that would be needed to cover the major
compaction with task manager's tasks.
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