Fix matchmaking bug with concurrency set to 0

[AttackingOrDefending]

Type of pull request:

• Bug fix
• Feature
• Other

Description:

The matchmaker would challenge other bots even when concurrency was set to 0, leading to the problems described in #1003. This PR fixes this.

Related Issues:

#1003

Checklist:

• I have read and followed the contribution guidelines.
• I have added necessary documentation (if applicable).
• The changes pass all existing tests.

Screenshots/logs (if applicable):

[MarkZH]

So, after this fix, setting challenge: concurrency: 0 results in no challenges being sent or accepted. In other words, the bot does nothing. Should we print a warning and exit in this case?

[MarkZH]

Something like this in config.py:

config_assert(CONFIG["challenge"]["concurrency"] > 0, "challenge.concurrency must be greater than zero to play any games.")

[AttackingOrDefending]

I used config_warn instead of config_assert because setting concurrency to 0 has some possible uses:

1. The bot's owner can create challenges through the lichess website and the bot will play the games (useful for testing).
2. After restarting lichess-bot, when you want your bot to just finish all the already started games.

[AttackingOrDefending]

Also, just to note, it is still possible to make a bot not accept any challenges but still create ones, by setting min_base to a value larger than max_base (e.g. 60 when max_base is 30).

[MarkZH]

I used config_warn instead of config_assert because setting concurrency to 0 has some possible uses:

1. The bot's owner can create challenges through the lichess website and the bot will play the games (useful for testing).

2. After restarting lichess-bot, when you want your bot to just finish all the already started games.

Point 1 makes sense. For point 2, the user can set quit_after_all_games_finish: true.

Also, just to note, it is still possible to make a bot not accept any challenges but still create ones, by setting min_base to a value larger than max_base (e.g. 60 when max_base is 30).

This should probably have a warning, as well as any other configuration with min_ and max_. But, that can be another PR.