Mpservice

- Improvements to the utilities in `mpservice.http_server`, esp regarding service shutdown.

- Requirement on `uvicorn` changes to `uvicorn[standard]`, which uses `uvloop`.

- Reworked error propagation in streamer; fixed bugs therein.
- Renamed `exception` to `remote_exception`.
- Corner-case bug fixes in `MPServer`.
- Increase queue size in `ConcurrentTransformer`.

- Refactor the `MPError` class, with a renaming to `RemoteException`.

- Refactor the `MPError` class.

- Relax version requirements on dependencies.