Directory structure pinning

I run an IPFS cluster installation which has a quite fast update cycle of its pin. I like to retain the history for a while, but not force all cluster members to hold the full history.

This way new members or members who have limited space are not required to fetch or hold the historical data, while they might still be fully available from some of the members.

To allow fast browsing in the directory structure it makes sense to hold it recursive without holding any file data.

Since UnixFS 1.0 requires having the first block (at least) of each file to create a listing with file sizes and types it would be nice to have a dedicated pin-mode for this which holds all refs of a UnixFS which are required for a listing of directories.

This functionality could also be interesting for other applications, where you want to retain the metadata of a directory structure on all clients, but only request the data from the storage nodes which is actually requested.