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

This is much more space efficient!

Logs/Presence.hs

@@ -16,7 +16,6 @@ module Logs.Presence (
 	addLog,
 	readLog,
 	logNow,
-	logThen,
 	currentLog,
 	currentLogInfo,
 	historicalLogInfo,
@@ -44,9 +43,6 @@ logNow s i = do
 	now <- liftIO getPOSIXTime
 	return $ LogLine now s i
 
-logThen :: POSIXTime -> LogStatus -> String -> Annex LogLine
-logThen t s i = return $ LogLine t s i
-
 {- Reads a log and returns only the info that is still in effect. -}
 currentLogInfo :: FilePath -> Annex [String]
 currentLogInfo file = map info <$> currentLog file