Fix ReplaySubject Terminal State Race Condition

[benjchristensen]

Fixes #1147 (comment) with change from @akarnokd

[benjchristensen]

Unit test to assert this is:

    @Test
    public void testRaceForTerminalState() {
        final List<Integer> expected = Arrays.asList(1);
        for (int i = 0; i < 1000000; i++) {
            TestSubscriber<Integer> ts = new TestSubscriber<Integer>();
            Observable.just(1).subscribeOn(Schedulers.computation()).cache().subscribe(ts);
            ts.awaitTerminalEvent();
            ts.assertReceivedOnNext(expected);
            ts.assertTerminalEvent();
        }
    }

I am including that test with a smaller loop size. If this bug were to return that test should non-deterministically fail, even with the smaller loop.

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