Store aid trackers inline in LowCountState

[edongashi]

Why do we have a list of pointers here: https://github.com/diffix/pg_diffix/blob/f605204371533719063552aed5e4d7b1739f720d/src/aggregation/low_count.c#L44

Why not store AidTrackerStates by value, avoiding the extra pallocs?

[cristianberneanu]

I think this was implemented so mainly to try to be consistent with state handling in other locations and to keep it easier in the future to make the state variable size, if the need arises.