Make JsonNode.toString() use shared ObjectMapper to produce valid json

[cowtowncoder]

Jackson 2.x has half-assed implementation of JsonNode.toString() which sort-of, mostly works, but:

1. Not quite, because
2. Implementation can not access standard JsonGenerator, so it is implemented with a simple, non-complete, code-duplicating writers

At first the plan was to actually make toString() produce visibly non-JSON output, to make it obvious that proper way to serialize JsonNodes is to do use ObjectMapper.writeValueAsString() (or equivalent).
But with #2176 we have a better way: we can produce valid JSON with standard/vanilla settings.
This should be convenient for many "casual JSON" use cases, and would achieve the goals of:

1. Valid JSON output, if it looks such
2. No more code duplication

[cowtowncoder]

Actually. Now that I think about this bit more, I think this CAN be made to work for 2.10 too.
Not by using globally shared, public JsonMapper, but rather creating a private, non-shared, but static-ly accessible instance. In addition will need to catch, re-throw IOException, but it should work just fine.

Will also need to add unit tests to verify now official support.