[SPARK-4688] Have a single shared network timeout in Spark

[SPARK-4688] Have a single shared network timeout in Spark
@rxin , how about this ? 

 val slaveTimeout = conf.getLong("spark.storage.blockManagerSlaveTimeoutMs",
    conf.getInt("spark.network.timeout", 
      math.max(
        conf.getInt("spark.executor.heartbeatInterval", 10000) * 3, 45000)/1000) * 1000)