Matchmaking should avoid challenging user that consistently declines challenges

[fabriciodosanjossilva]

After challenging a user via Matchmaking, if challenged user declines the challenge, linux-bot should "remember" that for awhile, avoiding annoying the user and wasting time. I'm getting declines quite frequently from bots that are not accepting challenges or have strict time restrictions.

As an implementation suggestion, this can be done as a "no contact" time window, where no challenge is to be sent to same user within this window. The window should start low (say 5 minutes) and increase exponentially (doubling on every next decline). It would be great if this window (for every user) is persistent, even after lichess-bot restart.

It seems that public user data does not contain challenge restrictions. If this information is available somewhere else, challenges that will be declined because of this public restrictions could be avoided upfront.