Separate the instantiation and initialisation of the stores

Add a new `init()` method to the `Store` class that is supposed to
handle all the effectful initialisation needed to set-up the store.
The constructor should remain side-effect free and just initialize the
c++ data structure.

The goal behind that is that we can create “dummy” instances of each
store to query static properties about it (the parameters it accepts for
example)

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

@@ -23,6 +23,12 @@ private:
 
 public:
 
+    HttpBinaryCacheStore(
+        const Params & params)
+        : HttpBinaryCacheStore("dummy", params)
+    {
+    }
+
     HttpBinaryCacheStore(
         const Path & _cacheUri,
         const Params & params)