Rework locking mechanism in blockchain-explorer. (#772)

Mainly because it was causing crash on Windows with error "unlock of unowned mutex".

blockchain-explorer/blockchain-explorer.cpp

@@ -103,23 +103,24 @@ class HttpQueryRunner {
         Self->finish(nullptr);
       }
     });
-    mutex_.lock();
     scheduler_ptr->run_in_context_external([&]() { func(std::move(P)); });
   }
   void finish(MHD_Response* response) {
+    std::unique_lock<std::mutex> lock(mutex_);
     response_ = response;
-    mutex_.unlock();
+    cond.notify_all();
   }
   MHD_Response* wait() {
-    mutex_.lock();
-    mutex_.unlock();
+    std::unique_lock<std::mutex> lock(mutex_);
+    cond.wait(lock, [&]() { return response_ != nullptr; });
     return response_;
   }
 
  private:
   std::function<void(td::Promise<MHD_Response*>)> func_;
-  MHD_Response* response_;
+  MHD_Response* response_ = nullptr;
   std::mutex mutex_;
+  std::condition_variable cond;
 };
 
 class CoreActor : public CoreActorInterface {