After acquiring the _compaction_state write lock,
select all sstables using get_candidates and register them
as compacting, then unlock the _compaction_state lock
to let regular compaction run in parallel.
Also, run major compaction in maintenance scheduling group.
We should separate the scheduling groups used for major compaction
from the the regular compaction scheduling group so that
the latter can be affected by the backlog tracker in case
backlog accumulates during a long running major compaction.
Fixes#10961Closes#10984
* github.com:scylladb/scylla:
compaction_manager: major_compaction_task: run in maintenance scheduling groupt
compaction_manager: allow regular compaction to run in parallel to major
34886ce1a1