There exists an edge case where a donor could be told to go into dropping after kPreparingToMirror. This is possible in this scenario:

1. A donor tells recipients via the oplog that writes are now distributed transactions.
2. The donor steps down or crashes.
3. The resharding operation has moved on to a state past mirroring while the donor was offline.

This ticket is to ensure that it won't break code for a donor to transition directly from kPreparingToMirror to kDropping.