[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