wrap FileWriter with BufferedWriter for better performance

Location: src/main/java/org/apache/flink/examples/java/relational/util/WebLogDataGenerator.java

The FileWriter.write() method is invoked multiple times in loops, which is bad to the performance of program. The decorator class BufferedWriter can be used to alleviate the impact of frequent IO operation with caches. Therefore, when the write() method is intensively used, the BufferedWriter is highly recommended and should be preferred.