Task manager task implementations of classes that cover
rewrite sstables keyspace compaction which can be start
through /storage_service/keyspace_compaction/ api.
Top level task covers the whole compaction and creates child
tasks on each shard.
Closes#12714
* github.com:scylladb/scylladb:
test: extend test_compaction_task.py to test rewrite sstables compaction
compaction: create task manager's task for rewrite sstables keyspace compaction on one shard
compaction: create task manager's task for rewrite sstables keyspace compaction
compaction: create rewrite_sstables_compaction_task_impl