Compaction beside the writer: a compactor attaches its copies to the
stored inode (FLAG_COMPACTION_PENDING and the inode object's metadata),
and the writer takes them in before its next publish of data.
runtime.take_up_on_open, Staging::parts_of, SEGMENT_FLAG_CLEANER.
Staging::flush_inode takes the compaction to write. btree-ondisk 0.20.3.