Xml2Json: should not collect JSON result in memory

Converting an XML to JSON collects the entire JSON output in memory first, before writing out the JSON.

If possible we should instead write the JSON to a MessageBuilder directly so that memory usage is reduced and garbage collection overhead minimized.

As part of this change, the JsonWriter class should get an option to format the JSON that is being written to keep a readable output, like the current JsonDocumentContainer now produces.

When done we can remove the JsonDocumentContainer/JsonElementContainer classes, because they are not used anywhere else.