This is in order to prepare for compatibility with the Alembic exporter.
That exporter is capable of writing object transforms and object data at
different (sub)frames.
The rename from `created_writers` to `used_writers` is necessary, as not
all created writers will be actually used in each iteration.
The Universal Scene Description (USD) exporter does not make use of
this.
Reviewed By: mont29
Differential Revision: https://developer.blender.org/D7670
69c3d9804f