benchmark the overhead caused by system versioning

Thoughts:

• inserts should work pretty much as before, little or no overhead (unless there are indexes and history size >> current data size)
• updates should have about the same (2x?) overhead in partitioned and non-partitioned case
• selects should have little or no overhead in partitioned case, and notable overhead in non-partitioned case (with history size >> current data size)
• deletes should have little overhead in non-partitioned case, and notable overhead (2x?) in partitioned case
• versioning by transaction id has more overhead as compared with versioning by timestamps