Recur.getDates() is very slow since #488

[db82407]

Describe the bug

Recur.getDates() has become very slow since #488, when the seed date has a timezone set.

To Reproduce

I've added a new test-case to RecurTest to demonstrate the problem in PR #507

    private static TimeZoneRegistry tzReg = TimeZoneRegistryFactory.getInstance().createRegistry();

    public RecurTest(Recur recur, Date seed, Date periodStart, Date periodEnd, Value value, int expectedCount, long maxTime) {
        this("testGetDatesMaxTime", recur, seed, periodStart, periodEnd, value);
        this.expectedCount = expectedCount;
        this.maxTime = maxTime;
    }

    public void testGetDatesMaxTime() {
        DateTime start = new DateTime(seed);
        start.setTimeZone(tzReg.getTimeZone("Europe/London"));
        Period period = new Period(new DateTime(periodStart), new DateTime(periodEnd));

        long t0 = System.currentTimeMillis();
        DateList dates = recur.getDates(start, period, value);
        long dt = System.currentTimeMillis() - t0;

        String message = String.format("maxTime exceeded %dms", maxTime);
        assertEquals(message, maxTime, Math.max(dt, maxTime));
        assertEquals(expectedCount, dates.size());
    }

    // check maxTime
    recur = new Recur("FREQ=WEEKLY;WKST=MO;UNTIL=20160901T230000;INTERVAL=1;BYDAY=TH");
    suite.addTest(new RecurTest(recur, new DateTime("20160414T100000"),
            new DateTime("20110713T213021"), new DateTime("20230713T213021"),
            Value.DATE_TIME, 21, 100));

Expected behavior

Recur.getDates() should complete in < 100ms.

Before #488 it typically took < 10ms

Screenshots

After #488 it now takes > 900ms!
junit.framework.AssertionFailedError: maxTime exceeded 100ms expected:<100> but was:<909>

Environment (please complete the following information):

• OS: Linux
• Java Version [8, 11]

Additional context

This test fails to demonstrate the problem.
PR #508 fixes this issue and when applied the test succeeds,