We need to verify that an unfinished 4.2-style two phase drop (defer drop in WT) could be handled nicely on startup of 4.0 binary.

If an 4.2-style two phase drop is unfinished on the shutdown of 4.2 binary, it means the first phase (dropping metadata in the catalog) is completed (committed but not checkpointed yet - metadata still exists from disk's perspective) but the second phase (dropping ident in WT) is not. And a "dropCollection" oplog entry is written (committed and persistent on disk in the form of WT journal).

On the startup of 4.0 binary, MongoDB will start with the last checkpoint which has the original collection and replay the oplog entries including that "dropCollection". The collection will be dropped with 4.0-style (old way) two phase drop mechanism.4.0 binary does not need to do anything related to the 4.2-style two phase drop in this case.

This applies to both primary and secondary nodes. This ticket is to write a downgrade test to verify the theory.