rethought distributed fsck; instead add activity.log and expire command

This is much more space efficient!

Logs/Activity.hs

@@ -0,0 +1,37 @@
+{- git-annex activity log
+ -
+ - Copyright 2015 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.Activity (
+	Log,
+	Activity(..),
+	recordActivity,
+	lastActivities,
+) where
+
+import Data.Time.Clock.POSIX
+
+import Common.Annex
+import qualified Annex.Branch
+import Logs
+import Logs.UUIDBased
+
+data Activity = Fsck
+	deriving (Eq, Read, Show, Enum, Bounded)
+
+recordActivity :: Activity -> UUID -> Annex ()
+recordActivity act uuid = do
+	ts <- liftIO getPOSIXTime
+	Annex.Branch.change activityLog $
+		showLog id . changeLog ts uuid (show act) . parseLog readish
+
+lastActivities :: Maybe Activity -> Annex (Log Activity)
+lastActivities wantact = parseLog onlywanted <$> Annex.Branch.get activityLog
+  where
+	onlywanted s = case readish s of
+		Just a | wanted a -> Just a
+		_ -> Nothing
+	wanted a = maybe True (a ==) wantact