libstore/remote: frame buffers, don't buffer frames

shoving a nar dump directly into a framed sink created a bunch of tiny
frames, each of which requires at least two syscalls to read. this can
lead to immense performance loss when using a daemon; we have seen 15%
in benchmarks on main and even more with async code involved ... oops.

Change-Id: I8529506e3de74d92834d1f4ee228dcaf32eb756c

lix/libstore/remote-store.cc

@@ -834,15 +834,15 @@ kj::Promise<Result<void>> RemoteStore::ConnectionHandle::withFramedStream(
     std::function<kj::Promise<Result<void>>(AsyncOutputStream & stream)> fun
 )
 try {
-    AsyncBufferedOutputStream to(stream);
-    AsyncFramedOutputStream sink(to);
+    AsyncFramedOutputStream framed(stream);
+    AsyncBufferedOutputStream sink(framed);
 
     // NOLINTNEXTLINE(cppcoreguidelines-avoid-capturing-lambda-coroutines)
     auto send = [&]() -> kj::Promise<Result<void>> {
         try {
             TRY_AWAIT(fun(sink));
-            TRY_AWAIT(sink.finish());
-            TRY_AWAIT(to.flush());
+            TRY_AWAIT(sink.flush());
+            TRY_AWAIT(framed.finish());
             co_return result::success();
         } catch (...) {
             handle.markBad();