Timestamp#forMillis(BigDecimal, Integer) susceptible to DoS

[raganhan]

calling longValue() on large exponent BigDecimals, e.g. 1e100000000 and 1e-100000000, is very expensive, each took ~2 minutes on my machine.

A solution is to check bounds and removing the fractional component before calling longValue()

Affected code:
https://github.com/amzn/ion-java/blob/master/src/software/amazon/ion/Timestamp.java#L689-L707

private Timestamp(BigDecimal millis, Integer localOffset)
    {
        if (millis == null) throw new NullPointerException("millis is null");

        long ms = millis.longValue();
        set_fields_from_millis(ms);

        this._precision = Precision.SECOND;
        int scale = millis.scale();
        if (scale <= -3) {
            this._fraction = null;
        }
        else {
            BigDecimal secs = millis.movePointLeft(3);
            BigDecimal secsDown = secs.setScale(0, RoundingMode.FLOOR);
            this._fraction = secs.subtract(secsDown);
        }
        this._offset = localOffset;
    }

That's the only occurrence of BigDecimal#longValue I found in ion-java

Jackson is dealing with a similar issue: FasterXML/jackson-databind#2141