Add `JsonNode.equals(Comparator<JsonNode>, JsonNode)` to support configurable/external equality comparison

[cowtowncoder]

Since it is difficult to make default JsonNode.equals(Object) work "perfectly" (cover all use cases users have), partly because there is no way to configure this handling (no way to pass context/config info, for example), it may make more sense to add a way to use custom Comparators.
The only real functionality in databinding, then, would be traversal of ArrayNodes and ObjectNodes, to handle structural part; but use passed-in comparator object for scalar value comparisons.

[cowtowncoder]

Ok: as per unit test:

    public void testCustomComparators() throws Exception
    {
        ObjectNode root1 = MAPPER.createObjectNode();
        root1.put("value", 5);
        ObjectNode root2 = MAPPER.createObjectNode();
        root2.put("value", 5.0);

        // default equals(): not strictly equal
        assertFalse(root1.equals(root2));

        // but. Custom comparator can make all the difference
        Comparator<JsonNode> cmp = new Comparator<JsonNode>() {
            @Override
            public int compare(JsonNode o1, JsonNode o2) {
                if (o1.equals(o2)) {
                    return 0;
                }
                if ((o1 instanceof NumericNode) && (o2 instanceof NumericNode)) {
                    double d1 = ((NumericNode) o1).asDouble();
                    double d2 = ((NumericNode) o2).asDouble();
                    if (d1 == d2) { // strictly equals because it's integral value
                        return 1;
                    }
                }
                return 0;
            }
        };
        // but with custom comparator, yes
        assertTrue(root1.equals(cmp, root2));

[cowtowncoder]

@fge Not sure if it's relevant any more, but I think you wanted more easily configurable comparison. This addition in 2.6 will allow defining simple scalar comparators, and handle Array/Object traversal.