Have the incremental upgrade process use less RAM

Originally created by @intrigeri on #6876 (Redmine)

As part of this work, we should update some hardcoded constants:

memory_needed and space_needed in Tails::IUK::Frontend (the very goal of this work is to decrease these values)
MIN_AVAILABLE_MEMORY in config/chroot_local-includes/usr/local/bin/tails-upgrade-frontend-wrapper (since #17152 (closed) was done, likely the Upgrader can work with less free memory)

Feature Branch: feature/15281-single-squashfs-diff, iuk:feature/11131-endless-upgrade, iuk:feature/15281-single-squashfs-diff

Edited May 21, 2020 by intrigeri

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