setdiscovery: document the '_updatesample' function This function is central in the sample building process, having it documented help code readability a lot.

setdiscovery: avoid calling any sample building if the undecided set is small If the length of undecided is smaller than the sample size, we can just request information for all of them. This conditional was previously handled by '_setupsample'. But '_setupsample' is in my opinion a problematic function with blurry semantics. Having this conditional explicitly earlier makes the code more explicit and moves us closer to removing this '_setupsample' function.

setdiscovery: delay sample building calls to gather them in a single place Some of the logic around sample building is duplicated in the sample builders, it would clean up thing to extract it in the top function, but this requires all codes to be in the same place. This changeset mostly exists to make the next one more clear.

setdiscovery: drop unused 'initial' argument for '_takequicksample' There is a single call site, and it is always using 'initial=True'. So we just drop the argument and the associated condition.