This patch adds the ranges_parallelism option to repair restful API.
Users can use this option to optionally specify the number of ranges
to repair in parallel per repair job to a smaller number than the Scylla
core calculated default max_repair_ranges_in_parallel.
Scylla manager can also use this option to provide more ranges (>N) in
a single repair job but only repairing N ranges_parallelism in parallel,
instead of providing N ranges in a repair job.
To make it safer, unlike the PR #4848, this patch does not allow user to
exceed the max_repair_ranges_in_parallel.
Fixes#4847
9b3fd9407b