As part of WT-5986, we added a script for building the latest stable version of WiredTiger (v4.2) in the develop source tree and alternating between running develop and v4.2 workgen on the same set of data files.

We added a workgen workload called multiversion.py which demonstrated this concept by setting the compatibility_version and doing a few inserts.

The next step is to flesh out that workload (maybe make a few variants) and make it resemble things that MongoDB does. For example, one of the multiversion involved moving a chunk between shards with manifested itself at the WiredTiger levels as entire pages of data being deleted at once.