Support multiple event files that contain events not in chronological order

Use case

Sometimes, a model can be iteratively trained on multiple workers (e.g., using mutliprocessing). In that case, there is an tfevent_* file for each of the workers. So when the model is trained on worker_1, the events are persisted in the tfevent_1 file. Then, the model training can be dedicated to a different worker (worker_2) and the events are persisted in the tfevent_2 file. For next iterations, the model can be trained on worker_1 again, which means that the tfevent_1 is used.

The issue is that the we end up having tfevent files that contain events in non-chronological order:

tfevent_1 file:

• event 1
• event 3

tfevent_2 file:

• event 2

And rustboard fails to read them properly.

This is not a bug since this is a documented behaviour: https://github.com/tensorflow/tensorboard/blob/f45fdd667a63d0e6b4aac471556583e5f2b7dac2/tensorboard/data/server/logdir.rs#L36

Feature request

Allow rustboard to parse data in partially-chronological order.

Start by reading the documented event-ordering behavior in tensorboard/data/server/logdir.rs and then inspect rustboard's event-file parsing path. Reproduce the worker-file ordering described in the issue; done means rustboard can parse multiple event files whose events are only partially chronological.