src: use EnqueueMicrotask for tls writes

Instead of using SetImmediate, use EnqueueMicrotask as it appears to
be significantly more performant in certain cases and even in the
optimal case yields roughly 10% higher throughput.

src/tls_wrap.cc

@@ -292,12 +292,10 @@ void TLSWrap::EncOut() {
   NODE_COUNT_NET_BYTES_SENT(write_size_);
 
   if (!res.async) {
-    HandleScope handle_scope(env()->isolate());
-
     // Simulate asynchronous finishing, TLS cannot handle this at the moment.
-    env()->SetImmediate([](Environment* env, void* data) {
+    env()->isolate()->EnqueueMicrotask([](void* data) {
       static_cast<TLSWrap*>(data)->OnStreamAfterWrite(nullptr, 0);
-    }, this, object());
+    }, this);
   }
 }