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/binary-cache-store.cc

@@ -22,7 +22,8 @@
 namespace nix {
 
 BinaryCacheStore::BinaryCacheStore(const Params & params)
-    : Store(params)
+    : BinaryCacheStoreConfig(params)
+    , Store(params)
 {
     if (secretKeyFile != "")
         secretKey = std::unique_ptr<SecretKey>(new SecretKey(readFile(secretKeyFile)));