Index checkpoint files incrementally

[`index_checkpoint_files`](https://github.com/invenia/Checkpoints.jl/blob/92837561306a115df38aa388ad3e394372b41960/src/indexing.jl#L141) walks a given path to find checkpoint files, and organises the segments of each checkpoint path into tags.

One might want to call this many times on the same top-level checkpoint directory, to analyse checkpoint data while the program is running and new checkpoints are added. For example, if a checkpoint is made at regular time intervals, with the timestamp used as a tag.

If there are a lot of checkpoint files (e.g. 100s), walking the whole path becomes a big waste. One could index a subdirectory of the top-level checkpoint directory, but then not all of the tags would be found, because tags are part of the path.

Is there a way to update the checkpoint index incrementally, based on diffs in the file tree? For example, if I want to reindex per timestep, it only searches the checkpoints for that timestep and adds them to an existing index, but still knows all of the tags.