libutil: disallow AsyncCollect relocations

some promises capture `this`.  we could also allocate a shared state,
but this thing doesn't really need to ever be moved anyway. so there.

Change-Id: I50b5c44684a8ab4e984b1323de21f97ace4a864a

src/libutil/async-collect.hh

@@ -63,6 +63,9 @@ public:
     {
     }
 
+    // oneDone promises capture `this`
+    KJ_DISALLOW_COPY_AND_MOVE(AsyncCollect);
+
     kj::Promise<std::optional<Item>> next()
     {
         if (remaining == 0 && results.empty()) {