[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.

Author: zsxwing <zsxwing@gmail.com>

Closes #3378 from zsxwing/SPARK-4505 and squashes the following commits:

4abdbba [zsxwing] Add a ClassTag parameter to reduce the memory usage of CompactBuffer[T] when T is a primitive type

core/src/main/scala/org/apache/spark/util/collection/CompactBuffer.scala

@@ -17,6 +17,8 @@
 
 package org.apache.spark.util.collection
 
+import scala.reflect.ClassTag
+
 /**
  * An append-only buffer similar to ArrayBuffer, but more memory-efficient for small buffers.
  * ArrayBuffer always allocates an Object array to store the data, with 16 entries by default,
@@ -25,7 +27,7 @@ package org.apache.spark.util.collection
  * entries than that. This makes it more efficient for operations like groupBy where we expect
  * some keys to have very few elements.
  */
-private[spark] class CompactBuffer[T] extends Seq[T] with Serializable {
+private[spark] class CompactBuffer[T: ClassTag] extends Seq[T] with Serializable {
   // First two elements
   private var element0: T = _
   private var element1: T = _
@@ -34,7 +36,7 @@ private[spark] class CompactBuffer[T] extends Seq[T] with Serializable {
   private var curSize = 0
 
   // Array for extra elements
-  private var otherElements: Array[AnyRef] = null
+  private var otherElements: Array[T] = null
 
   def apply(position: Int): T = {
     if (position < 0 || position >= curSize) {
@@ -45,7 +47,7 @@ private[spark] class CompactBuffer[T] extends Seq[T] with Serializable {
     } else if (position == 1) {
       element1
     } else {
-      otherElements(position - 2).asInstanceOf[T]
+      otherElements(position - 2)
     }
   }
 
@@ -58,7 +60,7 @@ private[spark] class CompactBuffer[T] extends Seq[T] with Serializable {
     } else if (position == 1) {
       element1 = value
     } else {
-      otherElements(position - 2) = value.asInstanceOf[AnyRef]
+      otherElements(position - 2) = value
     }
   }
 
@@ -72,7 +74,7 @@ private[spark] class CompactBuffer[T] extends Seq[T] with Serializable {
       curSize = 2
     } else {
       growToSize(curSize + 1)
-      otherElements(newIndex - 2) = value.asInstanceOf[AnyRef]
+      otherElements(newIndex - 2) = value
     }
     this
   }
@@ -139,7 +141,7 @@ private[spark] class CompactBuffer[T] extends Seq[T] with Serializable {
           newArrayLen = Int.MaxValue - 2
         }
       }
-      val newArray = new Array[AnyRef](newArrayLen)
+      val newArray = new Array[T](newArrayLen)
       if (otherElements != null) {
         System.arraycopy(otherElements, 0, newArray, 0, otherElements.length)
       }
@@ -150,9 +152,9 @@ private[spark] class CompactBuffer[T] extends Seq[T] with Serializable {
 }
 
 private[spark] object CompactBuffer {
-  def apply[T](): CompactBuffer[T] = new CompactBuffer[T]
+  def apply[T: ClassTag](): CompactBuffer[T] = new CompactBuffer[T]
 
-  def apply[T](value: T): CompactBuffer[T] = {
+  def apply[T: ClassTag](value: T): CompactBuffer[T] = {
     val buf = new CompactBuffer[T]
     buf += value
   }