[ruby/rack] Use autotune for database connections

[p8]

Decrease the KB_PER_WORKER as this is a small application. Puma threads should not be more than 32 threads, but this might even be decreased more:

+--------+---------+------+------+-----+-----+-------+--------------+
|threads |plaintext|update|  json|   db|query|fortune|weighted_score|
+--------+---------+------+------+-----+-----+-------+--------------+
|      32|   145031| 11936|122017|27001|18156|  23712|          1500|
|       2|   126527| 11878|141483|33525|20515|  27725|          1591|
|       1|   126701| 12923|133573|31402|19468|  24894|          1617|
|       5|    34445| 15098|103356|29189|23860|  22518|          1802|
|       5|    34440| 16363|114751|29820|20933|  24349|          1845|
+--------+---------+------+------+-----+-----+-------+--------------+