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.

Assistant.hs

@@ -66,7 +66,10 @@
  - 	Uses the ScanRemotes map.a
  - Thread 17: PairListener
  - 	Listens for incoming pairing traffic, and takes action.
- - Thread 18: WebApp
+ - Thread 18: ConfigMonitor
+ - 	Triggered by changes to the git-annex branch, checks for changed
+ - 	config files, and reloads configs.
+ - Thread 19: WebApp
  - 	Spawns more threads as necessary to handle clients.
  - 	Displays the DaemonStatus.
  -
@@ -114,6 +117,7 @@ import Assistant.Changes
 import Assistant.Commits
 import Assistant.Pushes
 import Assistant.ScanRemotes
+import Assistant.BranchChange
 import Assistant.TransferQueue
 import Assistant.TransferSlots
 import Assistant.Threads.DaemonStatus
@@ -128,6 +132,7 @@ import Assistant.Threads.MountWatcher
 import Assistant.Threads.NetWatcher
 import Assistant.Threads.TransferScanner
 import Assistant.Threads.TransferPoller
+import Assistant.Threads.ConfigMonitor
 #ifdef WITH_WEBAPP
 import Assistant.WebApp
 import Assistant.Threads.WebApp
@@ -174,6 +179,7 @@ startAssistant assistant daemonize webappwaiter = withThreadState $ \st -> do
 			transferqueue <- newTransferQueue
 			transferslots <- newTransferSlots
 			scanremotes <- newScanRemoteMap
+			branchhandle <- newBranchChangeHandle
 #ifdef WITH_WEBAPP
 			urlrenderer <- newUrlRenderer
 #endif
@@ -187,7 +193,7 @@ startAssistant assistant daemonize webappwaiter = withThreadState $ \st -> do
 #endif
 				, assist $ pushThread st dstatus commitchan pushmap
 				, assist $ pushRetryThread st dstatus pushmap
-				, assist $ mergeThread st dstatus transferqueue
+				, assist $ mergeThread st dstatus transferqueue branchhandle
 				, assist $ transferWatcherThread st dstatus transferqueue
 				, assist $ transferPollerThread st dstatus
 				, assist $ transfererThread st dstatus transferqueue transferslots
@@ -197,6 +203,7 @@ startAssistant assistant daemonize webappwaiter = withThreadState $ \st -> do
 				, assist $ netWatcherThread st dstatus scanremotes
 				, assist $ netWatcherFallbackThread st dstatus scanremotes
 				, assist $ transferScannerThread st dstatus scanremotes transferqueue
+				, assist $ configMonitorThread st dstatus branchhandle
 				, watch $ watchThread st dstatus transferqueue changechan
 				]
 			waitForTermination