Mixed-mode timestamps are defined by doing writes to a document with valid timestamps, followed by a write with a 0 (globally visible) timestamp. The intent is for such a write to reset the visibility chain, but the complexity comes when the update isn't visible to readers due to it's transaction ID. In which case older updates remain accessible.

We should add the ability to log via __wt_msg when a mixed mode operation happens. With the intent of having MongoDB enable that logging to help with diagnostics.