honor preferred content settings of cluster nodes

Except when no nodes want a file, it has to be stored somewhere, so
store it on all. Which is not really desirable, but neither is having to
pick one.

ProtoAssociatedFile deserialization is rather broken, and this could
possibly affect preferred content expressions that match on filenames.

The inability to roundtrip whitespace like tabs and newlines through is
not a problem because preferred content expressions can't be written
that match on whitespace such as a tab. For example:

joey@darkstar:~/tmp/bench/z>git-annex wanted  origin-node2 'exclude=*CTRL-VTab*'
wanted origin-node2
git-annex: Parse error: Parse failure: near "*"

But, the filtering of control characters could perhaps be a problem. I think
that filtering is now obsolete, git-annex has comprehensive filtering of
control characters when displaying filenames, that happens at a higher level.
However, I don't want to risk a security hole so am leaving in that filtering
in ProtoAssociatedFile deserialization for now.

doc/todo/git-annex_proxies.mdwn

@@ -26,9 +26,6 @@ In development on the `proxy` branch.
 
 For June's work on [[design/passthrough_proxy]], remaining todos:
 
-* On upload to cluster, send to nodes where its preferred content, and not
-  to other nodes. Unless no nodes prefer it, then what?
-
 * Getting a key from a cluster currently always selects the lowest cost
   remote, and always the same remote if cost is the same. Should
   round-robin amoung remotes, and prefer to avoid using remotes that
@@ -110,3 +107,6 @@ For June's work on [[design/passthrough_proxy]], remaining todos:
 
 * Avoid `git-annex sync --content` etc from operating on cluster nodes by
   default since syncing with a cluster implicitly syncs with its nodes. (done)
+
+* On upload to cluster, send to nodes where its preferred content, and not
+  to other nodes. (done)