[PP-1] Avoid account switching when calculating permissions in AccessPolicyProcessor::processAccessPolicies()

Problem/Motivation

If permissions need to be calculated for an account other than the current user, AccessPolicyProcessor::processAccessPolicies() has to switch the current user to that account in order to calculate and cache the data correctly.

The account switching led to problems with fibers as seen in #3576074: Current user is changed unexpectedly. While a workaround was implemented there, ideally switching the current user should not need to be done at all. This should be possible once #2628870: Access result caching per user(.permissions) does not check for correct user is done, so that issue is a blocker here.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet