[server] Support multiple ancestors in fetchDescendantsForUpdate
[server] Support multiple ancestors in fetchDescendantsForUpdate

Summary: This optimization was hard to do in the past, but since I am going to rework `updateDescendantPermissions` to iterate by depth, it becomes trivial.

Test Plan: Will be tested in combination with following diffs

Reviewers: palys-swm, atul

Reviewed By: palys-swm

Subscribers: KatPo, Adrian

Differential Revision: https://phabricator.ashoat.com/D1375

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal