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.

stack.yaml

@@ -14,8 +14,9 @@ extra-deps:
   - configurator-pg-0.2.6@sha256:cd9b06a458428e493a4d6def725af7ab1ab0fef678fbd871f9586fc7f9aa70be,2849
   - hasql-dynamic-statements-0.3.1.1@sha256:2cfe6e75990e690f595a87cbe553f2e90fcd738610f6c66749c81cc4396b2cc4,2675
   - hasql-implicits-0.1.0.4@sha256:0848d3cbc9d94e1e539948fa0be4d0326b26335034161bf8076785293444ca6f,1361
-  - hasql-pool-0.5.2.2@sha256:b56d4dea112d97a2ef4b2749508c0ca646828cb2d77b827e8dc433d249bb2062,2438
   - lens-aeson-1.1.3@sha256:52c8eaecd2d1c2a969c0762277c4a8ee72c339a686727d5785932e72ef9c3050,1764
   - optparse-applicative-0.16.1.0@sha256:418c22ed6a19124d457d96bc66bd22c93ac22fad0c7100fe4972bbb4ac989731,4982
   - protolude-0.3.2@sha256:2a38b3dad40d238ab644e234b692c8911423f9d3ed0e36b62287c4a698d92cd1,2240
   - ptr-0.16.8.2@sha256:708ebb95117f2872d2c5a554eb6804cf1126e86abe793b2673f913f14e5eb1ac,3959
+  - git: https://github.com/PostgREST/hasql-pool.git
+    commit: 4d462c4d47d762effefc7de6c85eaed55f144f1d