* Draft of checkpoint freezer DB
* Fix bugs
* Adjust root iterators for checkpoint database
* Fix freezer state lookups with no slot hint
* Fix split comment
* Use "restore point" to refer to frozen states
* Resolve some FIXMEs
* Configurable slots per restore point
* Document new freezer DB functions
* Fix up StoreConfig
* Fix new test for merge
* Document SPRP default CLI flag, clarify tests
