TarMK compaction

The TarMK would benefit from periodic "compact" operations that would traverse and recreate (parts of) the content tree in order to optimize the storage layout. More specifically, such compaction would:

• Optimize performance by increasing locality and reducing duplication, both of which improve the effectiveness of caching.
• Allow the garbage collector to release more unused disk space by removing references to segments where only a subset of content is reachable.