[SPARK-6556][Core] Fix wrong parsing logic of executorTimeoutMs and...

[SPARK-6556][Core] Fix wrong parsing logic of executorTimeoutMs and checkTimeoutIntervalMs in HeartbeatReceiver

The current reading logic of `executorTimeoutMs` is:
```Scala
private val executorTimeoutMs = sc.conf.getLong("spark.network.timeout",
    sc.conf.getLong("spark.storage.blockManagerSlaveTimeoutMs", 120)) * 1000
```
So if `spark.storage.blockManagerSlaveTimeoutMs` is 10000 and `spark.network.timeout` is not set, executorTimeoutMs will be 10000 * 1000. But the correct value should have been 10000.

`checkTimeoutIntervalMs` has the same issue.

This PR fixes them.

Author: zsxwing <zsxwing@gmail.com>

Closes #5209 from zsxwing/SPARK-6556 and squashes the following commits:

6a0a411 [zsxwing] Fix docs
c7d5422 [zsxwing] Add comments for executorTimeoutMs and checkTimeoutIntervalMs
ccd5147 [zsxwing] Fix wrong parsing logic of executorTimeoutMs and checkTimeoutIntervalMs in HeartbeatReceiver

core/src/main/scala/org/apache/spark/HeartbeatReceiver.scala

@@ -49,12 +49,17 @@ private[spark] class HeartbeatReceiver(sc: SparkContext, scheduler: TaskSchedule
 
   // executor ID -> timestamp of when the last heartbeat from this executor was received
   private val executorLastSeen = new mutable.HashMap[String, Long]
-  
-  private val executorTimeoutMs = sc.conf.getLong("spark.network.timeout", 
-    sc.conf.getLong("spark.storage.blockManagerSlaveTimeoutMs", 120)) * 1000
-  
-  private val checkTimeoutIntervalMs = sc.conf.getLong("spark.network.timeoutInterval",
-    sc.conf.getLong("spark.storage.blockManagerTimeoutIntervalMs", 60)) * 1000
+
+  // "spark.network.timeout" uses "seconds", while `spark.storage.blockManagerSlaveTimeoutMs` uses
+  // "milliseconds"
+  private val executorTimeoutMs = sc.conf.getOption("spark.network.timeout").map(_.toLong * 1000).
+    getOrElse(sc.conf.getLong("spark.storage.blockManagerSlaveTimeoutMs", 120000))
+
+  // "spark.network.timeoutInterval" uses "seconds", while
+  // "spark.storage.blockManagerTimeoutIntervalMs" uses "milliseconds"
+  private val checkTimeoutIntervalMs =
+    sc.conf.getOption("spark.network.timeoutInterval").map(_.toLong * 1000).
+      getOrElse(sc.conf.getLong("spark.storage.blockManagerTimeoutIntervalMs", 60000))
   
   private var timeoutCheckingTask: Cancellable = null