message: core: make LazyPlugin thread-safe
  Before this, we could potentially create multiple plugins and overwrite
  each other.
diff: #pragma once
  #include <memory>
  +#include <mutex>
  #include <mavsdk.h>
  @@ -12,6 +13,7 @@ public:
  Plugin* maybe_plugin()
  {
  + std::lock_guard<std::mutex> lock(_mutex);
  if (_plugin == nullptr) {
  if (_mavsdk.systems().empty()) {
  return nullptr;
  @@ -24,6 +26,7 @@ public:
  private:
  Mavsdk& _mavsdk;
  std::unique_ptr<Plugin> _plugin{};
  + std::mutex _mutex{};
  };
  } // namespace mavsdk::mavsdk_server
  