* Separate calculation of hash value from the lookup.
  * Split build_hash_table() into two functions.
  * Change lookup_hash_entry() to return AggStatePerGroup. That's all
    the caller needed, anyway.

These changes are to support the upcoming Disk-based Hash Aggregation
work.

Discussion: https://postgr.es/m/31f5ab871a3ad5a1a91a7a797651f20e77ac7ce3.camel%40j-davis.com