Add writing tests for locust

[genematx]

Adding a script to test writing load with locust.

Checklist

• Add a Changelog entry
• Add the ticket number which this PR closes to the comment section

[gwbischof]

Looks good, did you test it?

[danielballan]

It may be better to replay pre-generated document streams. A couple benefits:

• Remove issue with the dueling Locust and RunEngine event loops.
• Reduce dependency footprint. (This pulls in Ophyd, which pulls in libca, and a whole host of potential complexity there.)
• It can run faster and produce maximum load, especially if the document streams are pre-loaded into RAM.