Use RemoteStore to open connection for proxying daemon

Removes duplicate websocket opening code, and also means we should be
able to to ssh-ssh-... daemon relays, not just uds-uds-... ones.

src/libstore/remote-store.hh

@@ -102,8 +102,6 @@ public:
 
     void flushBadConnections();
 
-protected:
-
     struct Connection
     {
         AutoCloseFD fd;
@@ -119,6 +117,8 @@ protected:
 
     ref<Connection> openConnectionWrapper();
 
+protected:
+
     virtual ref<Connection> openConnection() = 0;
 
     void initConnection(Connection & conn);