Mpservice

- Finetune to `EnsembleServer`: ensemble elements could be `RemoteException`
objects, i.e. failure of one ensemble component will not halt or invalidate
the other components.

- By default, `MPServer` uses the new, custom queue type `Unique` for faster
buildup of batches for servlets. Removed `ZeroQueue` and `BasicQueue`.
- Simplified timeout in `mpserver`. `EnqueueTimeout` is gone; `TotalTimeout` is renamed
`Timeout`. The parameters `enqueue_timeout` and `total_timeout` are combined
into `timeout`.
- `ServerProcess` gets new parameter `ctx` for multiprocessing context.

- Added alternative multiprocessing queues, namely `BasicQueue`, `FastQueue` and `ZeroQueue`,
in an attempt to improve service speed, expecially batching.
This topic is still under experimentation. `ZeroQueue` is not ready for use.
- Changed socket encoder from 'orjson' to 'pickle'.
- Removed `max_queue_size` parameter of `MPServer`; use unbounded queues.
- `MPServer` parameter `cpus` renamed to `main_cpu` with default value `0`.

- Minor fine-tuning and documentation touch-ups.

- Rewrote `socket` to be fully based on `asyncio.streams`.
- Refactored socket server side to make its usage similar to a web app.
- Refactored `_streamer` for much improved robustness and simplicity.
- Streamer API changes, mainly:
1. use context manager;
2. operations modify the hosting object in-place, hence eliminating the need to assign
intermediate transforms to new names.
- `Streamer.transform` now accepts both sync and async functions as the "operator".
- Improved `MPServer.stream` and `SocketClient.stream`; removed `MPServer.stream2`.
- Rewrote `RemoteException`, especially for pickle-safety.
- `SocketServer` shutdown handling.
- Removed `MPServer.{start, stop}`; use `__enter__/__exit__`.
- Removed some contents of `util`.

- Add async-based `MPServer.stream2`.
- Improve printing of RemoteException in `__exit__`.
- SocketServer creates directory path as needed at startup.
- Small improvements.