repair: Reduce parallelism in repair_ranges
repair: Reduce parallelism in repair_ranges

We currently repair all the ranges in parallel.

1) All the ranges will contend for parallelism_semaphore, instead of
processing multiple ranges in parallel and calculating the sub ranges
(which take memory) for each range in parallel, we can handle the ranges
one bye one.

We could have enough parallelism because the checksum are calucated on
all the shards.

2) If for some reason the repair failed, if we handle ranges 1 by 1, we
can log which range of repair is successful. Next time, we can ignore
them. If we start ranges in parallel, it has a high chance, no single
range is completed because all the ranges are on going.

Refs #1912

duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla