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/configs/expected/no-defaults-with-db.config

@@ -5,7 +5,6 @@ db-extra-search-path = "public,extensions,private"
 db-max-rows = 1000
 db-plan-enabled = true
 db-pool = 1
-db-pool-timeout = 100
 db-pre-request = "test.custom_headers"
 db-prepared-statements = false
 db-root-spec = "root"