If a billing plan changes, create Tlog records for cutover event

[tmarshall]

If a user changes billing plans (not possible yet, though) then the system should record a tlog event for any running instances up until that point. This does not need to be done for build events.

The cutover means we can bill up to that point for previous runs, and then after that it could bill for the new rates.

Need to ensure that the cover will work and be honored.

[tmarshall]

need to migrate this issue