Refactor the printing logic in compaction::formatted_sstables_list
out to sstables::to_string(const shared_sstable&, bool include_origin)
and operator<<(const shared_sstable) on top of it.
So that we can easily print std::vector<shared_sstable>
from compaction_manager in the next patch.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>