add ConfigMonitor thread

Monitors git-annex branch for changes, which are noticed by the Merger
thread whenever the branch ref is changed (either due to an incoming push,
or a local change), and refreshes cached config values for modified config
files.

Rate limited to run no more often than once per minute. This is important
because frequent git-annex branch changes happen when files are being
added, or transferred, etc.

A primary use case is that, when preferred content changes are made,
and get pushed to remotes, the remotes start honoring those settings.
Other use cases include propigating repository description and trust
changes to remotes, and learning when a remote has added a new special
remote, so the webapp can present the GUI to enable that special remote
locally.

Also added a uuid.log cache. All other config files already had caches.

Logs/Trust.hs

@@ -6,11 +6,13 @@
  -}
 
 module Logs.Trust (
+	trustLog,
 	TrustLevel(..),
 	trustGet,
 	trustSet,
 	trustPartition,
 	lookupTrust,
+	trustMapLoad,
 	trustMapRaw,
 ) where
 
@@ -65,27 +67,29 @@ trustPartition level ls
 		candidates <- trustGet level
 		return $ partition (`elem` candidates) ls
 
-{- Read the trustLog into a map, overriding with any
- - values from forcetrust or the git config. The map is cached for speed. -}
+{- trustLog in a map, overridden with any values from forcetrust or
+ - the git config. The map is cached for speed. -}
 trustMap :: Annex TrustMap
-trustMap = do
-	cached <- Annex.getState Annex.trustmap
-	case cached of
-		Just m -> return m
-		Nothing -> do
-			overrides <- Annex.getState Annex.forcetrust
-			logged <- trustMapRaw
-			configured <- M.fromList . catMaybes
-				<$> (mapM configuredtrust =<< remoteList)
-			let m = M.union overrides $ M.union configured logged
-			Annex.changeState $ \s -> s { Annex.trustmap = Just m }
-			return m
+trustMap = maybe trustMapLoad return =<< Annex.getState Annex.trustmap
+
+{- Loads the map, updating the cache, -}
+trustMapLoad :: Annex TrustMap
+trustMapLoad = do
+	overrides <- Annex.getState Annex.forcetrust
+	logged <- trustMapRaw
+	configured <- M.fromList . catMaybes
+		<$> (mapM configuredtrust =<< remoteList)
+	let m = M.union overrides $ M.union configured logged
+	Annex.changeState $ \s -> s { Annex.trustmap = Just m }
+	return m
 	where
 		configuredtrust r =
 			maybe Nothing (\l -> Just (Types.Remote.uuid r, l)) <$>
 				maybe Nothing readTrustLevel
 					<$> getTrustLevel (Types.Remote.repo r)
 
+{- Does not include forcetrust or git config values, just those from the
+ - log file. -}
 trustMapRaw :: Annex TrustMap
 trustMapRaw = simpleMap . parseLog (Just . parseTrustLog)
 	<$> Annex.Branch.get trustLog