Checkpointer

Durable storage for graph threads: each thread's latest checkpoint, the pending writes recorded against it, and its durable event log. One store owns all three so that a commit is atomic.

Implementations must be safe to call from several threads, and must:

• apply a Commit entirely or not at all;
• accept a new checkpoint only when its parent is the thread's latest checkpoint id (GraphError.CheckpointConflict otherwise), and only pending writes naming the resulting latest checkpoint (GraphError.InvalidCommit);
• number a thread's events 1, 2, 3, ... in commit order, inside the commit, never reusing a number - including after compaction or a failed commit;
• keep events until compactEvents removes them, and report the earliest sequence still available when asked to replay from before it (GraphError.ReplayUnavailable).

Fencing a commit with a run-claim token is Stage 2; the parent check is its precursor.