Use class template argument deduction for locks

src/test/jtx/impl/WSClient.cpp

@@ -284,7 +284,7 @@ private:
         auto m = std::make_shared<msg>(
             std::move(jv));
         {
-            std::lock_guard<std::mutex> lock(m_);
+            std::lock_guard lock(m_);
             msgs_.push_front(m);
             cv_.notify_all();
         }
@@ -297,7 +297,7 @@ private:
     void
     on_read_done()
     {
-        std::lock_guard<std::mutex> lock(m0_);
+        std::lock_guard lock(m0_);
         b0_ = true;
         cv0_.notify_all();
     }