Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

modules/gdnative/android/android_gdn.cpp

@@ -48,7 +48,7 @@ extern "C" {
 
 JNIEnv *GDAPI godot_android_get_env() {
 #ifdef __ANDROID__
-	return ThreadAndroid::get_env();
+	return get_jni_env();
 #else
 	return nullptr;
 #endif