Mpservice

- `mpservice.mpserver.Worker` gets new method `stream` for some special use cases.

- `mpservice.mpserver.Worker.__init__` loses parameter `batch_size_log_cadence` and gets new parameter `cpu_affinity`,
which also becomes an attribute of the worker instance.

- Removed some deprecated code.
- `mpservice.mpserver.Worker` gets new method `cleanup`.

- Bug fix in `mpservice.multiprocessing.Pool`.
- Finetune about shutdown of the background threads supporting `mpservice.multiprocessing.SpawnProcess`.

- Minor tweak on `mpserver` detail.

It happens in one application that some requests get stuck in the server "backlog", whereas by design
this should never happen (in reasonable time every input unit flows through the system and comes out of
the output queue, at which time clears its bookkeeping entry in the backlog). The tweak hopefully helps
debugging this situation.

- Bug fix in ``ServerBacklogFull`` class definition.