Coalesce token ranges sent in repair jobs

[Michal-Leszczynski]

As mentioned in the https://github.com/scylladb/scylla-enterprise/issues/4006, it might be beneficial to always send more token ranges per job than max_repair_ranges_in_parallel (e.g. the greater of 10 * max_repair_ranges_in_parallel and 5%-10% of all ranges owned by repaired replica set). It allows for better concurrency (the whole job won't be blocked from returning when only 1 token range is still being repaired). Note that this should be safe to do with non-max intensity as well. It would require SM to use the ranges_parallelism repair param that limits used parallelism even when SM sends less ranges than max_repair_ranges_in_parallel.

This behavior could be controlled by an additional flag or repair config option in scylla-manager.yaml.
A side benefit would be to decrease the amount of clutter in SM logs.
In terms of testing, it would be good to see performance improvement on a cluster like: 2ds, 5 nodes each, keyspace with RF 3 in each dc, setup in which the repair indeed has to do some work (missing rows on some nodes).