Document and check that PgStat_HashKey has no padding

This change is a tighter rework of 7d85d87f4d5c, which tried to improve
the code so as it would work should PgStat_HashKey gain new fields that
create padding bytes.  However, the previous change is proving to not be
enough as some code paths of pgstats do not pass PgStat_HashKey by
reference (valgrind would warn when padding is added to the structure,
through a new field).

Per discussion, let's document and check that PgStat_HashKey has no
padding rather than try to complicate the code of pgstats so as it is
able to work around that.

This removes a couple of memset(0) calls that should not be required.
While on it, this commit adds a static assertion checking that no
padding is introduced in the structure, by checking that the size of
PgStat_HashKey matches with the sum of the size of all its fields.

The object ID part of the hash key is already 8 bytes, which should be
plenty enough already.  A comment is added to discourage the addition of
new fields.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0t9omat+HVSakJXwTMWvhpYFcAZb41RPWKwrKFUgmAFBQ@mail.gmail.com

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