Fix outdated CHUNKHDRSZ value in nodeAgg.c

Fix outdated CHUNKHDRSZ value in nodeAgg.c

CHUNKHDRSZ was defined as 16 bytes, which was true when that code went in,
but since c6e0fe1f2, 8 is a more accurate value.  Here we adjust it to use
sizeof(MemoryChunk), which is normally 8, or 16 for cassert builds.

c6e0fe1f2 first appeared in v16, so this is technically wrong in v16 up
to master, but let's apply this only to master as adjusting this does
influence the estimated number of batches in the aggregate costing code
and we don't want to cause plan instability in released versions.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAApHDvpMpRQvsTqZo3FinXkgytwxwF8sCyZm83xDj-1s_hLe+w@mail.gmail.com