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/UUID.hs

@@ -8,34 +8,38 @@
  -
  - uuid.log stores a list of known uuids, and their descriptions.
  -
- - Copyright 2010-2011 Joey Hess <joey@kitenet.net>
+ - Copyright 2010-2012 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
 module Logs.UUID (
+	uuidLog,
 	describeUUID,
 	recordUUID,
-	uuidMap
+	uuidMap,
+	uuidMapLoad
 ) where
 
 import qualified Data.Map as M
 import Data.Time.Clock.POSIX
 
+import Types.UUID
 import Common.Annex
+import qualified Annex
 import qualified Annex.Branch
 import Logs.UUIDBased
 import qualified Annex.UUID
 
 {- Filename of uuid.log. -}
-logfile :: FilePath
-logfile = "uuid.log"
+uuidLog :: FilePath
+uuidLog = "uuid.log"
 
 {- Records a description for a uuid in the log. -}
 describeUUID :: UUID -> String -> Annex ()
 describeUUID uuid desc = do
 	ts <- liftIO getPOSIXTime
-	Annex.Branch.change logfile $
+	Annex.Branch.change uuidLog $
 		showLog id . changeLog ts uuid desc . fixBadUUID . parseLog Just
 
 {- Temporarily here to fix badly formatted uuid logs generated by
@@ -76,14 +80,20 @@ recordUUID u = go . M.lookup u =<< uuidMap
 		go _ = noop
 		set = describeUUID u ""
 
+{- The map is cached for speed. -}
+uuidMap :: Annex UUIDMap
+uuidMap = maybe uuidMapLoad return =<< Annex.getState Annex.uuidmap
+
 {- Read the uuidLog into a simple Map.
  -
  - The UUID of the current repository is included explicitly, since
  - it may not have been described and so otherwise would not appear. -}
-uuidMap :: Annex (M.Map UUID String)
-uuidMap = do
-	m <- (simpleMap . parseLog Just) <$> Annex.Branch.get logfile
+uuidMapLoad :: Annex UUIDMap
+uuidMapLoad = do
+	m <- (simpleMap . parseLog Just) <$> Annex.Branch.get uuidLog
 	u <- Annex.UUID.getUUID
-	return $ M.insertWith' preferold u "" m
+	let m' = M.insertWith' preferold u "" m
+	Annex.changeState $ \s -> s { Annex.uuidmap = Just m' }
+	return m'
 	where
 		preferold = flip const