Break out quota usage refresh code from quota_reserve()

This moves the logic out of the refresh block into a private method.

This is part of a larger series to refactor the quota_reserve method.

Partial-Bug: #1369605

Change-Id: I3cbff688c7079c112ebd4551f9b30ba2be96c98c
Merge "Break out quota usage refresh code from quota_reserve()"