Data migration utility

[robfromboulder]

Introduce an offline utility program that copies all key/value data between two persistent pools, allowing customers to easily convert data between different persistent engines, or different major versions of the same persistent engine. (If you have data stored using "tree3", this migration utility could copy all data for use e.g. by the "cmap" engines)

This would be easy to build for any engine that supports iteration (which is nearly all of them).