Currently, we do not explicitly set a scheduling group for the schema
commitlog which causes it to run in the default scheduling group (called
"main"). However:
- It is important and significant enough that it should run in a
scheduling group that is separate from the main one,
- It should not run in the existing "commitlog" group as user writes may
sometimes need to wait for schema commitlog writes (e.g. read barrier
done to learn the schema necessary to interpret the user write) and we
want to avoid priority inversion issues.
Therefore, introduce a new scheduling group dedicated to the schema
commitlog.
Fixes: scylladb/scylladb#15566Closesscylladb/scylladb#18715
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