Replace condchan with contextcond

[Jille]

condchan seems to be no longer maintained and has a race condition (https://gitlab.com/jonas.jasas/condchan/-/issues/1) that's not been fixed in two years.

Note that you currently are not affected by the race condition, because you only use .Broadcast() and not .Signal(). I propose to switch anyway in case someone would start using .Signal() later.