Fixes for macOS

Building on macOS with the latest nixpkgs master and NixOS/nixpkgs#77147
fails.  It seems some `std::experimental` (optional) for instance are
not available as `experimental`, but are in `std`.  Also `toJSON` is
missing for `atomic< unsigned long long >`.

src/libhydra/db.hh

@@ -23,7 +23,7 @@ struct Connection : pqxx::connection
 
 class receiver : public pqxx::notification_receiver
 {
-    std::experimental::optional<std::string> status;
+    std::optional<std::string> status;
 
 public:
 
@@ -35,9 +35,9 @@ public:
         status = payload;
     };
 
-    std::experimental::optional<std::string> get() {
+    std::optional<std::string> get() {
         auto s = status;
-        status = std::experimental::nullopt;
+        status = std::nullopt;
         return s;
     }
 };