[SPARK-4505][Core] Add a ClassTag parameter to CompactBuffer[T]

Added a ClassTag parameter to CompactBuffer. So CompactBuffer[T] can create primitive arrays for primitive types. It will reduce the memory usage for primitive types significantly and only pay minor performance lost.

Here is my test code:
```Scala
  // Call org.apache.spark.util.SizeEstimator.estimate
  def estimateSize(obj: AnyRef): Long = {
    val c = Class.forName("org.apache.spark.util.SizeEstimator$")
    val f = c.getField("MODULE$")
    val o = f.get(c)
    val m = c.getMethod("estimate", classOf[Object])
    m.setAccessible(true)
    m.invoke(o, obj).asInstanceOf[Long]
  }

  sc.parallelize(1 to 10000).groupBy(_ => 1).foreach {
    case (k, v) =>
      println(v.getClass() + " size: " + estimateSize(v))
  }
```

Using the previous CompactBuffer outputed
```
class org.apache.spark.util.collection.CompactBuffer size: 313358
```

Using the new CompactBuffer outputed
```
class org.apache.spark.util.collection.CompactBuffer size: 65712
```

In this case, the new `CompactBuffer` only used 20% memory of the previous one. It's really helpful for `groupByKey` when using a primitive value.