Multi-node checkpoint/restart benchmark on Fronterra

We want to model a checkpoint/restart workload.

* 4 client nodes, with 48 ranks each (out of 56)
* Treat `${SCRATCH}` as the PFS, or final destination.
* Tiers are `/tmp`, which is an nvme, and `RAM`.
* Run Hermes as a daemon
* Run `ior -w` to simulate a checkpoint
* Run `ior -r` to simulate a restart
* For the baseline, the checkpoint phase will exit once the data is flushed to PFS, and the restart phase will read from PFS.
* Hermes will store the checkpoint in the hierarchy and we should see faster write and read.

Blocked by ~~#181~~, and #266.