title: Get Max & Min value for each key in the RDD
body: <pre><code>spark = SparkSession.builder.getOrCreate()
  sc = spark.sparkContext
  ssc = StreamingContext(sc , 10)
  rdd = ssc.sparkContext.parallelize(pd_binance)
  rdd.take(1)
  </code></pre>
  <p>Here is a small portion of the result:</p>
  <pre><code>[['0.02703300', '1.30900000'],
     ['0.02703300', '0.61800000'],
     ['0.02704600', '3.90800000'],
     ['0.02704700', '4.00000000'],
     ['0.02704700', '7.44600000']
  </code></pre>
  <p>And I want to get the max and minimum value for each key, how?</p>