This is only working for shading batches for the moment and only if some Custom data layer are not needed anymore.
The collection rate is hardcoded at 60 sec but could be exposed to the user.
This system can be extended and discard most unused batches in the future.
This commit is in prevision of removing BKE_MESH_BATCH_DIRTY_SHADING when changing shader parameters.
223f7e79f9