run_processor: do we really _always_ want to save_mets() ?

[bertsky]

https://github.com/OCR-D/core/blob/62cf25c3406144c9eb628605431e5aa3e27e8332/src/ocrd/processor/helpers.py#L160

Now that we have the METS Server option, we should reconsider this behaviour. It gets used in both the standard processor CLI (via ocrd_cli_wrap_processor) and in ocrd_network (via invoke_processor).

At least the latter should be able to opt out of intermediate METS serialization. (Should only be done when stopping the METS Server.)