SparkContext's local properties should be cloned when inherited

Currently, when a child thread inherits local properties from the parent thread, it gets a reference of the parent's local properties and uses them as default values.

The problem, however, is that when the parent changes the value of an existing property, the changes are reflected in the child thread! This has very confusing semantics, especially in streaming.

private val localProperties = new InheritableThreadLocal[Properties] {
  override protected def childValue(parent: Properties): Properties = new Properties(parent)
  override protected def initialValue(): Properties = new Properties()
}

Instead, we should make a clone of the parent properties rather than passing in a mutable reference.