Before these changes, we didn't specify which I/O scheduling
group commitlog instances in hinted handoff should use.
In this commit, we set it explicitly to the commitlog
scheduling group. The rationale for this choice is the fact
we don't want to cause a bottleneck on the write path
-- if hints are written too slowly, new incoming mutations
(NOT hints) might be rejected due to a too high number
of hints currently being written to disk; see
`storage_proxy::create_write_response_handler_helper()`
for more context.
(cherry picked from commit 6a7fb18b52)
Closesscylladb/scylladb#20094
5abd343cf9