Add a Config class to simplify adding configuration settings

The typical use is to inherit Config and add Setting<T> members:

  class MyClass : private Config
  {
    Setting<int> foo{this, 123, "foo", "the number of foos to use"};
    Setting<std::string> bar{this, "blabla", "bar", "the name of the bar"};

    MyClass() : Config(readConfigFile("/etc/my-app.conf"))
    {
      std::cout << foo << "\n"; // will print 123 unless overriden
    }
  };

Currently, this is used by Store and its subclasses for store
parameters. You now get a warning if you specify a non-existant store
parameter in a store URI.

src/libstore/remote-store.cc

@@ -43,7 +43,7 @@ template Paths readStorePaths(Store & store, Source & from);
 RemoteStore::RemoteStore(const Params & params)
     : Store(params)
     , connections(make_ref<Pool<Connection>>(
-            std::max(1, std::stoi(get(params, "max-connections", "1"))),
+            std::max(1, (int) maxConnections),
             [this]() { return openConnectionWrapper(); },
             [](const ref<Connection> & r) { return r->to.good() && r->from.good(); }
             ))