There is a bug where:
1. [th1] shardCollection releases distLocks (which were acquired here) here.
2. [th2] A dropCollection is now able to acquire the distlocks and proceed. Runs until here.
3. [th1] shardCollection writes chunks
4. [th2] dropCollection runs setShardVersion on all shards with requested version UNSHARDED. However, shardCollection has written chunks, so setShardVersion fails here

shardCollection needs to be still holding the distlocks when writing the chunks.