Currently renameCollection across databases:

1. takes a global lock
2. copies the data from the source collection to a temporary collection (with a different UUID) in the target database
3. atomically drops the source collection and renames the temp collection in the target db to its final non-temp name in the target db.

We can change this to:

1. Take a global clock
2. Copy the data from the source collection to a temporary collection (with a different UUID) in the target database
3. Atomically move the source collection to the drop-pending namespace, change its UUUID, rename the temp collection in the target db to its final non-temp name in the target db, and change the UUID on the dest collection to the UUID of the original source collection.