util-jackson: share PrettyPrinter properly

The `PrettyPrinter` configured for a Jackson `ObjectWriter` is shared between all downstream `JsonGenerator`s. To make this thread-safe (in order to prevent multiple generators from corrupting the shared indentation level), a class derived from `PrettyPrinter` can also inherit from `Instantiable`, which basically turns the `ObjectWriter`-level `PrettyPrinter` into a factory that produces a new `PrettyPrinter` for each generator.

Since `util-jackson`'s `ArrayElementsOnNewLinesPrettyPrinter` inherits from `DefaultPrettyPrinter`, which has state that must be protected from corruption, we need to abide by this factory pattern.

Differential Revision: https://phabricator.twitter.biz/D1167308

util-jackson/src/main/scala/com/twitter/util/jackson/ScalaObjectMapper.scala

@@ -412,7 +412,7 @@ object ScalaObjectMapper {
 
 private[jackson] object ArrayElementsOnNewLinesPrettyPrinter extends DefaultPrettyPrinter {
   _arrayIndenter = DefaultIndenter.SYSTEM_LINEFEED_INSTANCE
-  override def createInstance(): DefaultPrettyPrinter = this
+  override def createInstance(): DefaultPrettyPrinter = new DefaultPrettyPrinter(this)
 }
 
 /**