Add IT for maximum String value length

[tatu-at-datastax]

Most Document limits (maximum doc size, max number of properties, maximum number value length etc) are already covered by both unit and integration tests.

But it looks like maximum String length (16,000 characters) is currently only tested by unit tests. There have been reports that long (over 5kB) String values may be problematic with Storage-Attached Index updates, so let's add ITs (max allowed, too long) to cover this gap.