Bump pool size to workaround libpq locking.

- Pool size now matches the max concurrency of requests used by the benchmark. Many other framworks appear to do similar matching.
- Idea inspired by: haskell-servant/servant#651 (comment)
- This finally restores all performance regression caused by `114b1b8`. Additionally we now finally blow past the performance of master at `6250eb8`.

frameworks/Haskell/servant/src/ServantBench.hs

@@ -62,7 +62,7 @@ run port dbSettings = do
     Warp.run port $ serve api $ server pool gen
   where
     halfSecond = 0.5
-    settings = (30, halfSecond, dbSettings)
+    settings = (512, halfSecond, dbSettings)
 
 newtype QueryId = QueryId { unQueryId :: Int }
 instance FromHttpApiData QueryId where