MergingIterator#resetPriorityQueue is bogus.

[Honore Vasconcelos]

If MergingIterator#seekToFirst() or MergingIterator#seek(key) are called after starting using the iterator (after calling first #next()), MergingIterator start returning wrong values.

Root cause:
resetPriorityQueue() should clear queue before reusing it, other wise duplicate iterators will be inserted and used in next iterations

Following unit test reproduce the problem:

@Test
public void testName() throws Exception {
    InternalKeyComparator comparator = new InternalKeyComparator(new BytewiseComparator());
    InternalKey key1 = new InternalKey(Slices.wrappedBuffer(new byte[]{'1'}), 1, ValueType.VALUE);
    InternalKey key2 = new InternalKey(Slices.wrappedBuffer(new byte[]{'3'}), 2, ValueType.VALUE);
    InternalKey key3 = new InternalKey(Slices.wrappedBuffer(new byte[]{'2'}), 3, ValueType.VALUE);
    InternalKey key4 = new InternalKey(Slices.wrappedBuffer(new byte[]{'4'}), 4, ValueType.VALUE);
    ImmutableList<Map.Entry<InternalKey, Slice>> of1 = ImmutableList
            .of(
                    Maps.immutableEntry(key1, Slices.EMPTY_SLICE),
                    Maps.immutableEntry(key2, Slices.EMPTY_SLICE)
            );
    ImmutableList<Map.Entry<InternalKey, Slice>> of2 = ImmutableList
            .of(
                    Maps.immutableEntry(key3, Slices.EMPTY_SLICE),
                    Maps.immutableEntry(key4, Slices.EMPTY_SLICE)
            );
    ImmutableList<InternalIterator> of = ImmutableList.of(new InternalKeySliceAbstractSeekingIterator(of1, comparator), new InternalKeySliceAbstractSeekingIterator(of2, comparator));
    MergingIterator mergingIterator = new MergingIterator(of, comparator);
    assertEquals(mergingIterator.next().getKey(), key1);
    assertEquals(mergingIterator.next().getKey(), key3);
    mergingIterator.seekToFirst();
    assertEquals(mergingIterator.next().getKey(), key1);
    assertEquals(mergingIterator.next().getKey(), key3);
    assertEquals(mergingIterator.next().getKey(), key2);
    assertEquals(mergingIterator.next().getKey(), key4); //fail here! key2 is return a second time!
    assertFalse(mergingIterator.hasNext());


}

private static class InternalKeySliceAbstractSeekingIterator extends AbstractSeekingIterator<InternalKey, Slice> implements InternalIterator {
    int index = 0;
    private final List<Map.Entry<InternalKey, Slice>> entries;
    private final InternalKeyComparator comparator;

    public InternalKeySliceAbstractSeekingIterator(List<Map.Entry<InternalKey, Slice>> entries, InternalKeyComparator comparator) {
        this.entries = entries;
        this.comparator = comparator;
    }

    @Override
    protected void seekToFirstInternal() {
        index = 0;
    }

    @Override
    protected void seekInternal(InternalKey targetKey) {
        index = entries.size();
        for (int i = 0; i < entries.size(); i++) {
            Map.Entry<InternalKey, Slice> entry = entries.get(i);
            if (comparator.compare(entry.getKey(), targetKey) >= 0) {
                index = i;
            }
        }
    }

    @Override
    protected Map.Entry<InternalKey, Slice> getNextElement() {
        return index < entries.size() ? entries.get(index++) : null;
    }
}