Fishnet

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* Updated to Stockfish 12 NNUE.
* Fishnet is now distributed as a standalone binary instead of a Python module.
To update, uninstall fishnet 1.x (`pip uninstall fishnet`) and install the
new version. You can keep using your `fishnet.ini`.
* Removed `--threads-per-process`. Analysis is now always single-threaded for
reproducibility, parallelizing over positions instead. This also allows
finishing games more quickly, instead of starting to analyse multiple games
at the same time.
* Removed `--memory`. All clients will now use the same setting for
reproducibility.
* Removed `--stockfish-command` and `--engine-dir`.
[Reproducible Stockfish builds](https://github.com/niklasf/fishnet-assets)
for various CPU models now come bundled with the fishnet binary.
* Removed deprecated `--fixed-backoff`, `--no-fixed-backoff`,
and `--setoption`.

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* Lichess-hosted instances: Make `UCI_Elo` gradient steeper. Reintroduce depth
limits to limit resource consumption of low levels.

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* New command: Use `python -m fishnet systemd-user` to generate a systemd user
service file.
* New command: Use `python -m fishnet benchmark` to try running the engine
before getting a fishnet key.
* Fix process shutdown order with systemd.
* Fix race condition during shutdown on Python 2.7.
* Lichess-hosted instances: Use `UCI_Elo` instead of `Skill Level` and expand
the range significantly. Low skill levels should now play much weaker.

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* Reduce maximum move time from 20s to 6s. Clients that frequently hit this
limit should be stopped in favor of clients with better hardware.
* Support future proof constants `--user-backlog short` and
`--system-backlog long` (to be used instead of hardcoded durations).
* Fix some ignored command line flags during `python -m fishnet configure`
and on intial run.

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* Bring back `--threads-per-process`. Most contributors should not use this.

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* Option to join only if a backlog is building up. Added `--user-backlog`
and `--system-backlog` to configure threshold for oldest item in queue.
Run `python -m fishnet configure` to rerun the setup dialog.
* Slow clients no longer work on young user requested analysis jobs. The
threshold is continuously adjusted based on performance on other jobs.