checkpoint: rename committed types to persistent vocabulary

Type-level rename (no method or behavior changes yet): CommittedStore -> PersistentStore CommittedReader -> PersistentReader CommittedListReader -> PersistentListReader WriteCommittedOptions -> WriteOptions UpdateCommittedOptions -> UpdateOptions CommittedMetadata -> Metadata CommittedInfo -> CheckpointInfo

'Committed' collides with git's commit vocabulary (these records are literally stored as git commits); 'persistent' names the actual durability property. Method names and the Stores facade field are renamed in following commits.