This is similar to what physics baking is doing: invoking the operator
runs a background job, whereas executing blocks. This makes Python
scripts calling the Alembic import/export operators more predictable.
For backwardward compatibility with existing Python code the
`as_background_job` parameter still exists, which overrides the
behaviour chosen by INVOKE/EXECUTE.
Reviewers: brecht
Reviewed by: brecht
Differential revision: https://developer.blender.org/D4137/new/
196a5116aa