implement cluster.log

Not used yet. (Or tested.)

I did consider making the log start with the uuid of the node, followed
by the cluster uuid (or uuids). That would perhaps mean a smaller write
to the git-annex branch when adding a node, but overall the log file
would be larger, and it will be read and cached near to startup on most
git-annex runs.

Logs.hs

@@ -99,6 +99,7 @@ topLevelNewUUIDBasedLogs :: [RawFilePath]
 topLevelNewUUIDBasedLogs =
 	[ exportLog
 	, proxyLog
+	, clusterLog
 	]
 
 {- Other top-level logs. -}
@@ -158,6 +159,9 @@ exportLog = "export.log"
 proxyLog :: RawFilePath
 proxyLog = "proxy.log"
 
+clusterLog :: RawFilePath
+clusterLog = "cluster.log"
+
 {- This is not a log file, it's where exported treeishes get grafted into
  - the git-annex branch. -}
 exportTreeGraftPoint :: RawFilePath