nixos/postgresql: set Restart=always for postgresql.service

...including a slightly more careful config around restarts, i.e.

* We have intervals of 5 seconds between restarts instead of 100ms.

* If we exceed 5 start attempts in 5*120s (with 120s being the timeout),
  start job gets rate-limited and thus aborted. Do note that there are
  at most 5 start attempts allowed in ~625s by default. If the startup
  fails very quickly, either wait until the rate-limit is over or reset
  the counter using `systemctl reset-failed postgresql.service`.

* The interval of 625s (plus 5s of buffer) are automatically derived
  from RestartSec & TimeoutSec. Changing either will also affect
  StartLimitIntervalSec unless overridden with `mkForce`.

nixos/tests/postgresql/postgresql.nix

@@ -101,6 +101,12 @@ let
           machine.fail(check_count("SELECT * FROM sth;", 4))
           machine.succeed(check_count("SELECT xpath('/test/text()', doc) FROM xmltest;", 1))
 
+          with subtest("killing postgres process should trigger an automatic restart"):
+              machine.succeed("systemctl kill -s KILL postgresql")
+
+              machine.wait_until_succeeds("systemctl is-active postgresql.service")
+              machine.wait_until_succeeds("systemctl is-active postgresql.target")
+
           with subtest("Backup service works"):
               machine.succeed(
                   "systemctl start ${backupService}.service",