Break out over-quota processing from quota_reserve()

This moves the over-quota processing out of quota_reserve and into a
private method.

This completes the refactor of the quota_reserve method to isolate the
large chunks of logic into separate private methods.

Closes-Bug: #1369605

Change-Id: Ic910ea042dfe50209f79928e61b3b34b4c734e73
Merge "Break out over-quota processing from quota_reserve()"