Separate store configs from the implems

Rework the `Store` hierarchy so that there's now one hierarchy for the
store configs and one for the implementations (where each implementation
extends the corresponding config). So a class hierarchy like

```
StoreConfig-------->Store
    |                 |
    v                 v
SubStoreConfig----->SubStore
    |                 |
    v                 v
SubSubStoreConfig-->SubSubStore
```

(with virtual inheritance to prevent DDD).

The advantage of this architecture is that we can now introspect the configuration of a store without having to instantiate the store itself

src/libstore/http-binary-cache-store.cc

@@ -7,7 +7,12 @@ namespace nix {
 
 MakeError(UploadToHTTP, Error);
 
-class HttpBinaryCacheStore : public BinaryCacheStore
+struct HttpBinaryCacheStoreConfig : virtual BinaryCacheStoreConfig
+{
+    using BinaryCacheStoreConfig::BinaryCacheStoreConfig;
+};
+
+class HttpBinaryCacheStore : public BinaryCacheStore, public HttpBinaryCacheStoreConfig
 {
 private:
 
@@ -23,16 +28,11 @@ private:
 
 public:
 
-    HttpBinaryCacheStore(
-        const Params & params)
-        : HttpBinaryCacheStore("dummy", params)
-    {
-    }
-
     HttpBinaryCacheStore(
         const Path & _cacheUri,
         const Params & params)
         : BinaryCacheStore(params)
+        , HttpBinaryCacheStoreConfig(params)
         , cacheUri(_cacheUri)
     {
         if (cacheUri.back() == '/')
@@ -176,6 +176,6 @@ protected:
 
 };
 
-static RegisterStoreImplementation<HttpBinaryCacheStore> regStore;
+static RegisterStoreImplementation<HttpBinaryCacheStore, HttpBinaryCacheStoreConfig> regStore;
 
 }