availability-distribution: fix active leaf processing at session boundaries

[slumber]

As noticed by @eskimor

Here we are not actually using the SessionInfo for the leaf, but the SessionInfo for the leaf's child. Therefore, at session boundaries we will look up the backing group in the wrong session. grimacing

(link to the comment)

This is not crucial since such candidates are unlikely to be included anyway, but it's best to keep the behavior consistent.
The actual session index for the activated leaf is obtained here, so it's sufficient to extract it and refactor requester methods to have it as a parameter.

[eskimor]

Thanks @slumber !