Handle possible security problem with ObjectNode, HashMap, String.hashCode()

[cowtowncoder]

(see [https://github.com/FasterXML/jackson-core/issues/21] for related problem)

Given that it is easy to fabricate collisions for String.hashCode() (since JDK's impl allows substring-replacement style attacks very easily), ObjectNode is prone to attacks.
Since JsonParser use has been fixed (Issue-21), it is necessary to simialrly address the issue of use of HashMap with default String hashCode().

The big problem here is the performance: not so much that of cost of calculating hashCode() alone, but lack of caching of that value (JDK aggressively caches hashCode on first access).

We should probably do similar two-part solution here as earlier: make it possible to detect abnormal (super high collision list) cases and error out; but also improve hash code used as much as possible.

[cowtowncoder]

I think latest JDK 7 and 8 versions use work-around for case of HashMap / HashSet (wish I had a link at hand), so closing this.