assistant unused file handling

Make sanity checker run git annex unused daily, and queue up transfers
of unused files to any remotes that will have them. The transfer retrying
code works for us here, so eg when a backup disk remote is plugged in,
any transfers to it are done. Once the unused files reach a remote,
they'll be removed locally as unwanted.

If the setup does not cause unused files to go to a remote, they'll pile
up, and the sanity checker detects this using some heuristics that are
pretty good -- 1000 unused files, or 10% of disk used by unused files,
or more disk wasted by unused files than is left free. Once it detects
this, it pops up an alert in the webapp, with a button to take action.

TODO: Webapp UI to configure this, and also the ability to launch an
immediate cleanup of all unused files.

This commit was sponsored by Simon Michael.

debian/changelog

@@ -18,6 +18,15 @@ git-annex (5.20140118) UNRELEASED; urgency=medium
     preferred content expressions.
   * Client, transfer, incremental backup, and archive repositories
     now want to get content that does not yet have enough copies.
+  * Client, transfer, and source repositories now do not want to retain
+    unused file contents.
+  * assistant: Checks daily for unused file contents, and when possible
+    moves them to a repository (such as a backup repository) that
+    wants to retain them.
+  * assistant: annex.expireunused can be configured to cause unused
+    file contents to be deleted after some period of time.
+  * webapp: Nudge user to see if they want to expire old unused file
+    contents when a lot of them seem to be piling up in the repository.
   * repair: Check git version at run time.
   * assistant: Run the periodic git gc in batch mode.