Overhaul store subclassing

We embrace virtual the rest of the way, and get rid of the
`assert(false)` 0-param constructors.

We also list config base classes first, so the constructor order is
always:

  1. all the configs
  2. all the stores

Each in the same order

src/libstore/remote-store.cc

@@ -77,8 +77,8 @@ void write(const Store & store, Sink & out, const std::optional<ContentAddress>
 
 /* TODO: Separate these store impls into different files, give them better names */
 RemoteStore::RemoteStore(const Params & params)
-    : Store(params)
-    , RemoteStoreConfig(params)
+    : RemoteStoreConfig(params)
+    , Store(params)
     , connections(make_ref<Pool<Connection>>(
             std::max(1, (int) maxConnections),
             [this]() {