Task manager compaction tasks that cover compaction group
compaction need access to compaction_manager::tasks.
To avoid circular dependency and be able to rely on forward
declaration, task needs to be moved out of compaction manager.
To avoid naming confusion compaction_manager::task is renamed.
Closes#13226
* github.com:scylladb/scylladb:
compaction: use compaction namespace in compaction_manager.cc
compaction: rename compaction::task
compaction: move compaction_manager::task out of compaction manager
compaction: move sstable_task definition to source file
8167f11a23