perf: avoid taking mutex on already-resolved promises

Author: Kha

Diff for: src/runtime/object.cpp

@@ -852,6 +852,10 @@ class task_manager {
     }
 
     void resolve(lean_task_object * t, object * v) {
+        if (t->m_value) {
+            dec(v);
+            return;
+        }
         unique_lock<mutex> lock(m_mutex);
         if (t->m_value) {
             lock.unlock(); // `dec(v)` could lead to `deactivate_task` trying to take the lock