1. Previously pg_stat_monitor had a `topquery` and `topqueryid` field, but it was only a sample:
it showed one of the top queries executing the specific query.

With this change, the same entry executed by two different functions will result in two entries in the statistics table.

2. This also fixes a bug where the content of these field disappeared for every second query executed:
previously the update function changed topqueryid to `0` if it was non zero, and changed it to the proper id when it was 0.
This resulted in an alternating behavior, where for every second executed query the top query disappeared.

After these changes, the top query is always shown.

3. The previous implementation also leaked dsa memory used to store the parent queries. This is now also fixed.

* dsa_free changed to assert as it can never happen
* restructured the ifs to be cleaner
  Note: kept the two-level ifs, as that makes more sense with the assert
  Note: didn't convert nested_level checks to macro, as it is used differently at different parts of the code