Use wpi::mutex instead of std::mutex. (#254)

This uses a priority-aware mutex on Linux platforms.

src/main/native/cpp/InstanceImpl.h

@@ -10,8 +10,8 @@
 
 #include <atomic>
 #include <memory>
-#include <mutex>
 
+#include "support/mutex.h"
 #include "support/UidVector.h"
 
 #include "ConnectionNotifier.h"
@@ -52,7 +52,7 @@ class InstanceImpl {
   static std::atomic<int> s_default;
   static std::atomic<InstanceImpl*> s_fast_instances[10];
   static wpi::UidVector<std::unique_ptr<InstanceImpl>, 10> s_instances;
-  static std::mutex s_mutex;
+  static wpi::mutex s_mutex;
 };
 
 }  // namespace nt