add timestamps to unused log files

This will be used in expiring old unused objects. The timestamp is when it
was first noticed it was unused.

Backwards compatability: It supports reading old format unused log files.
The old version of git-annex will ignore lines in log files written by the
new version, so the worst interop problem would be git annex dropunused not
knowing some numbers that git-annex unused reported.

Test.hs

@@ -683,7 +683,7 @@ test_unused env = intmpclonerepoInDirect env $ do
   where
 	checkunused expectedkeys desc = do
 		git_annex env "unused" [] @? "unused failed"
-		unusedmap <- annexeval $ Logs.Unused.readUnusedLog ""
+		unusedmap <- annexeval $ Logs.Unused.readUnusedMap ""
 		let unusedkeys = M.elems unusedmap
 		assertEqual ("unused keys differ " ++ desc)
 			(sort expectedkeys) (sort unusedkeys)