balanced preferred content and --rebalance

This all works fine. But it doesn't check repository sizes yet, and
without repository size checking, once a repository gets full, there
will be no other repository that will want its files.

Use of sha2 seems unncessary, probably alder2 or md5 or crc would have
been enough. Possibly just summing up the bytes of the key mod the number
of repositories would have sufficed. But sha2 is there, and probably
hardware accellerated. I doubt very much there is any security benefit
to using it though. If someone wants to construct a key that will be
balanced onto a given repository, sha2 is certianly not going to stop
them.

doc/todo/git-annex_proxies.mdwn

@@ -30,9 +30,17 @@ Planned schedule of work:
 
 ## work notes
 
-* onward to balanced preferred content! But it depends on 
-  [[track_free_space_in_repos_via_git-annex_branch]] so that will be the
-  first task.
+* balanced= and fullybalanced= need to limit the set of repositories to
+  ones with enough free space to contain a key.
+
+* Add `git-annex size` command.
+
+* Implement [[track_free_space_in_repos_via_git-annex_branch]]
+
+## completed items for August's work on balanced preferred content
+
+* Balanced preferred content basic implementation, including --rebalance
+  option.
 
 ## completed items for August's work on git-annex proxy support for exporttre