Memory usage 100X what it should be for HashAgg

The transition state values are occupying 128K when they should be about 1.2K (for default values of ``regsize`` and ``log2m``).

This happens because of: https://github.com/citusdata/postgresql-hll/blob/master/src/hll.c#L3115 allocating ``sizeof(multiset_t)``, which is sized to be worse than the worst case. This allocated value is then returned by ``hll_add_trans0``.