upgrade to fork of hasql-pool 0.7.2 (fixes #2401)

This version of hasql-pool is a simplified rewrite that doesn't use
the resource-pool package. The major API changes are that idle
connections are no longer timed out (and the corresponding setting
is gone), and that `release` makes the pool unusable, where it used
to remain usable and only flushed idle connections.

We depend on a PostgREST fork of 0.7.2 that gives us reliable
flushing, compare https://github.com/PostgREST/hasql-pool/pull/1

- hasql-pool 0.7 removes timing out of idle connections, so
  this change removes the db-pool-timeout option.
  Given that we were typically running with very high
  timeout settings, I don't anticipate the lack of timeout
  to introduce new issues, though we might want to consider
  introducing some retry-logic down the line when we
  encounter connection failures.
- See https://github.com/PostgREST/postgrest/issues/2422 for a
  discussion on depending on a forked dependency. Besides adding
  the dependency to the nix overlay, we're also adding it to
  stack.yaml and a new cabal.project to allow stack/cabal users
  to build the project.

nix/overlays/haskell-packages.nix

@@ -31,6 +31,16 @@ let
       #
       # To get the sha256:
       #   nix-prefetch-url --unpack https://github.com/<owner>/<repo>/archive/<commit>.tar.gz
+
+      hasql-pool = lib.dontCheck (
+        prev.callCabal2nix "hasql-pool"
+          (super.fetchFromGitHub {
+            owner = "PostgREST";
+            repo = "hasql-pool";
+            rev = "4d462c4d47d762effefc7de6c85eaed55f144f1d"; # master as of 2022-08-26
+            sha256 = "sha256-UwX1PynimrQHm1KCs4BQXMwYYY3h4T5UAkgtEJ0EZQQ=";
+          })
+          { });
     } // extraOverrides final prev;
 in
 {