Saving an allocation for running the functor
as a task in the switched-to scheduling group.
Also, switch to the desired scheduling group at
the beginning of the task so that the higher level logic,
like getting the list of sstables to compact
will be performed under the desired scheduling group,
not only the compaction code itself.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
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