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.

test/io/test_io.py

@@ -123,7 +123,6 @@ def defaultenv(baseenv):
         "PGRST_DB_CONFIG": "true",
         "PGRST_LOG_LEVEL": "info",
         "PGRST_DB_POOL": "1",
-        "PGRST_DB_POOL_TIMEOUT": "1",
     }
 
 
@@ -139,7 +138,6 @@ def metapostgrest():
         "PGRST_DB_CONFIG": "true",
         "PGRST_LOG_LEVEL": "info",
         "PGRST_DB_POOL": "1",
-        "PGRST_DB_POOL_TIMEOUT": "1",
     }
     with run(env=env) as postgrest:
         yield postgrest
@@ -995,7 +993,6 @@ def test_pool_size(defaultenv, metapostgrest):
         assert delta > 1 and delta < 1.5
 
 
-@pytest.mark.xfail(reason="issue #2401")
 def test_change_statement_timeout_held_connection(defaultenv, metapostgrest):
     "Statement timeout changes take effect immediately, even with a request outliving the reconfiguration"