synchronize LCS getEstimatedTasks to avoid CME

patch by jbellis; reviewed by slebresne for CASSANDRA-4255

CHANGES.txt

@@ -1,6 +1,8 @@
 1.0.11
+ * synchronize LCS getEstimatedTasks to avoid CME (CASSANDRA-4255)
  * ensure unique streaming session id's (CASSANDRA-4223)
 
+
 1.0.10
  * fix maxTimestamp to include row tombstones (CASSANDRA-4116)
  * avoid streaming empty files with bulk loader if sstablewriter errors out

src/java/org/apache/cassandra/db/compaction/LeveledManifest.java

@@ -445,7 +445,7 @@ public class LeveledManifest
         return generations[i];
     }
 
-    public int getEstimatedTasks()
+    public synchronized int getEstimatedTasks()
     {
         long tasks = 0;
         for (int i = generations.length - 1; i >= 0; i--)