`JsonPointer` quadratic memory use: OOME on deep inputs

Adding https://github.com/nst/JSONTestSuite in https://github.com/rallyhealth/weePickle/pull/106 uncovered that JsonPointer memory usage is quadratic over depth. -Xmx8g is insufficient to create a JsonPointer for 100k opening arrays without an OutOfMemoryError.

JsonPointer seems to be a linked list where each node contains an _asString field containing the full path to that node.

Minimal test to repro the issue: 3764ff4b1a8dc2090dec57da5dc0cc2c7f8f520b

    // such as https://github.com/nst/JSONTestSuite/blob/master/test_parsing/n_structure_100000_opening_arrays.json
    public void testDeepJsonPointer() throws Exception {
        int DEPTH = 100000;
        String INPUT = new String(new char[DEPTH]).replace("\0", "[");
        JsonParser parser = createParser(MODE_READER, INPUT);
        try {
            while (true) {
                parser.nextToken();
            }
        } catch (Exception e) {
            JsonStreamContext parsingContext = parser.getParsingContext();
            JsonPointer jsonPointer = parsingContext.pathAsPointer(); // OOME
            String pointer = jsonPointer.toString();
            String expected = new String(new char[DEPTH - 1]).replace("\0", "/0");
            assertEquals(expected, pointer);
        }
    }

[cowtowncoder]

Finally found time to fix this; will be in 2.14.0 final.

[cowtowncoder]

@pjfanning If we needed cache Caffeine is great. But in this case unfortunately caches would not really work, I think. Problem is rather that of how to remove eager construction and retaining of String representation while still allowing efficient operation for common usage.

Caches are highly problematic for this usage due to various reasons, including lack of clear ownership (JsonPointer instances should not have references back to any larger retained objects, all caches need to be owned by life-cycled entities like either JsonFactory or ObjectMapper and so on).