Introduce 64-bit hash functions with a 64-bit seed.

This will be useful for hash partitioning, which needs a way to seed
the hash functions to avoid problems such as a hash index on a hash
partitioned table clumping all values into a small portion of the
bucket space; it's also useful for anything that wants a 64-bit hash
value rather than a 32-bit hash value.

Just in case somebody wants a 64-bit hash value that is compatible
with the existing 32-bit hash values, make the low 32-bits of the
64-bit hash value match the 32-bit hash value when the seed is 0.

Robert Haas and Amul Sul

Discussion: http://postgr.es/m/CA+Tgmoafx2yoJuhCQQOL5CocEi-w_uG4S2xT0EtgiJnPGcHW3g@mail.gmail.com

src/backend/utils/adt/pg_lsn.c

@@ -179,6 +179,12 @@ pg_lsn_hash(PG_FUNCTION_ARGS)
 	return hashint8(fcinfo);
 }
 
+Datum
+pg_lsn_hash_extended(PG_FUNCTION_ARGS)
+{
+	return hashint8extended(fcinfo);
+}
+
 
 /*----------------------------------------------------------
  *	Arithmetic operators on PostgreSQL LSNs.