BlockFileLoader: stream() call streamBuffers()

`stream()` is `streamBuffers()` with a `map()` operation to
read a buffer into a Block. Make this more clear by removing
duplicated code.

core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java

@@ -204,8 +204,7 @@ public class BlockFileLoader implements Iterable<Block> {
     }
 
     public Stream<Block> stream() {
-        return files.stream()
-                .flatMap(this::fileBlockStream)
+        return streamBuffers()
                 .map(serializer::makeBlock);
     }