Mpservice

- Removed async capabilities from `Stream`.
- New class `AsyncStream`, providing the async capabilities of the previous `Stream`.
This new class is not available in `mpservice.streamer`; it's only in `mpservice._streamer` because I'm still not sure
whether async streaming is useful.

- `IterableQueue.put` gets keyword arg `timeout`.
- `IterableQueue.get` becomes `_get`.
- New functions `mpservice.streamer.{fifo_stream, async_fifo_stream}`.
- Simplified implementations of classes `mpservice._streamer.*Parmapper*` and methods `mpservice.mpserver.{Server, AsyncServer}.stream` using these functions.
- `mpservice.streamer.Stream.parmapper` parameter `num_workers` was renamed `concurrency` because the meaning of the latter is more correct in async contexts.
- `mpservice.mpserver.Server.stream` and `mpservice.mpserver.AsyncServer.stream` lost parameter `to_stop`.

- Bug fix in `IterableQueue` when using a multiprocessing SimpleQueue.
- Refinements and enhancements to `IterableQueue`.
- `mpservice.mpserver.{Server, AsyncServer}.stream` gets a new parameter `preprocess`.
- New classes `ProcessRunner` and `ProcessRunnee` in `mpservice._streamer`.

- Added new class `mpservice.streamer.IterableQueue`.
- The directory structure under `mpservice.multiprocessing` better mirrors that of the standard lib.
- `Parmapper` and `Server.stream` get new parameter `to_stop` to allow the stream to be stopped by external forces.

- Finetune `mpservice.multiprocessing.SpawnProcess` regarding termination, background threads, and error handling.
- `mpservice.multiprocessing.Queue` gets property `maxsize`, making it aligned with `queue.Queue`, which has attribute `maxsize`.

- Finetune `mpservice.multiprocessing.server_process`:
- support pickling of `ServerProcess` objects.
- buf fix regarding custom authkey to `ServerProcess`