Fixed occasional ConcurrentModificationException in the block archive reader.

src/main/java/org/qortal/repository/BlockArchiveReader.java

@@ -145,6 +145,7 @@ public class BlockArchiveReader {
     }
 
     private String getFilenameForHeight(int height) {
+        synchronized (this.fileListCache) {
             Iterator it = this.fileListCache.entrySet().iterator();
             while (it.hasNext()) {
                 Map.Entry pair = (Map.Entry) it.next();
@@ -161,6 +162,7 @@ public class BlockArchiveReader {
                     return filename;
                 }
             }
+        }
 
         return null;
     }