Include chunk overhead in hash table entry size estimate.

 Include chunk overhead in hash table entry size estimate.
 
 Don't try to be precise about it, just use a constant 16 bytes of
 chunk overhead. Being smarter would require knowing the memory context
 where the chunk will be allocated, which is not known by all callers.
 
 Discussion: https://postgr.es/m/20200325220936.il3ni2fj2j2b45y5@alap3.anarazel.de