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.

postgrest.cabal

@@ -88,7 +88,7 @@ library
                     , hasql                     >= 1.4 && < 1.6
                     , hasql-dynamic-statements  >= 0.3.1 && < 0.4
                     , hasql-notifications       >= 0.1 && < 0.3
-                    , hasql-pool                >= 0.5 && < 0.6
+                    , hasql-pool                >= 0.7.2 && < 0.8
                     , hasql-transaction         >= 1.0.1 && < 1.1
                     , heredoc                   >= 0.2 && < 0.3
                     , http-types                >= 0.12.2 && < 0.13
@@ -226,7 +226,7 @@ test-suite spec
                     , bytestring        >= 0.10.8 && < 0.12
                     , case-insensitive  >= 1.2 && < 1.3
                     , containers        >= 0.5.7 && < 0.7
-                    , hasql-pool        >= 0.5 && < 0.6
+                    , hasql-pool        >= 0.7.2 && < 0.8
                     , hasql-transaction >= 1.0.1 && < 1.1
                     , heredoc           >= 0.2 && < 0.3
                     , hspec             >= 2.3 && < 2.9
@@ -269,7 +269,7 @@ test-suite querycost
                     , contravariant     >= 1.4 && < 1.6
                     , hasql             >= 1.4 && < 1.6
                     , hasql-dynamic-statements >= 0.3.1 && < 0.4
-                    , hasql-pool        >= 0.5 && < 0.6
+                    , hasql-pool        >= 0.7.2 && < 0.8
                     , hasql-transaction >= 1.0.1 && < 1.1
                     , heredoc           >= 0.2 && < 0.3
                     , hspec             >= 2.3 && < 2.9