[SPARK-20558][CORE] clear InheritableThreadLocal variables in SparkContext when stopping it

## What changes were proposed in this pull request?

To better understand this problem, let's take a look at an example first:
```
object Main {
  def main(args: Array[String]): Unit = {
    var t = new Test
    new Thread(new Runnable {
      override def run() = {}
    }).start()
    println("first thread finished")

    t.a = null
    t = new Test
    new Thread(new Runnable {
      override def run() = {}
    }).start()
  }

}

class Test {
  var a = new InheritableThreadLocal[String] {
    override protected def childValue(parent: String): String = {
      println("parent value is: " + parent)
      parent
    }
  }
  a.set("hello")
}
```
The result is:
```
parent value is: hello
first thread finished
parent value is: hello
parent value is: hello
```

Once an `InheritableThreadLocal` has been set value, child threads will inherit its value as long as it has not been GCed, so setting the variable which holds the `InheritableThreadLocal` to `null` doesn't work as we expected.

In `SparkContext`, we have an `InheritableThreadLocal` for local properties, we should clear it when stopping `SparkContext`, or all the future child threads will still inherit it and copy the properties and waste memory.

This is the root cause of https://issues.apache.org/jira/browse/SPARK-20548

 , which creates/stops `SparkContext` many times and finally have a lot of `InheritableThreadLocal` alive, and cause OOM when starting new threads in the internal thread pools.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #17833 from cloud-fan/core.

(cherry picked from commit b946f316)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>