Factor out commonality between WorkerProto::Basic{Client,Server}Connection

This also renames clientVersion and daemonVersion to the more correct
protoVersion (since it's the version agreed to by both sides).

src/libstore/daemon.hh

@@ -10,8 +10,8 @@ enum RecursiveFlag : bool { NotRecursive = false, Recursive = true };
 
 void processConnection(
     ref<Store> store,
-    FdSource & from,
-    FdSink & to,
+    FdSource && from,
+    FdSink && to,
     TrustedFlag trusted,
     RecursiveFlag recursive);