Before entries can be added to the index, a writer should obtain a unique id.
The methods to add new entries to the index require this id as an argument.
The application can implement a custom function to map the writer object to a string. That string will be used to register or look up an id in the index.
The caller must not hold writer's GST_OBJECT_LOCK, as the default resolver may call functions that take the object lock as well, and the lock is not recursive.
writer
GST_OBJECT_LOCK
