Exists returns whether a checkpoint with the given key eixsts.

Insert inserts a checkpoint with the given key and TTL.

ReadAllForTesting reads all checkpoints for testing, e.g.

SetForTesting replaces the set of stored checkpoints to match the given set.

SortKeys sorts keys in place in ascending order by Project, ResourceId, ProcessId, Uniquifier.

Checkpoint records that a point in a process was reached.

Key is the primary key of a checkpoint.