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/uds-remote-store.cc

@@ -15,6 +15,9 @@ namespace nix {
 
 UDSRemoteStore::UDSRemoteStore(const Params & params)
     : StoreConfig(params)
+    , LocalFSStoreConfig(params)
+    , RemoteStoreConfig(params)
+    , UDSRemoteStoreConfig(params)
     , Store(params)
     , LocalFSStore(params)
     , RemoteStore(params)