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/local-store.hh

@@ -95,7 +95,7 @@ public:
 
 private:
 
-    Setting<bool> requireSigs{(Store*) this,
+    Setting<bool> requireSigs{(StoreConfig*) this,
         settings.requireSigs,
         "require-sigs", "whether store paths should have a trusted signature on import"};