SERVER-48882 made it so that when using directoryperdb, newly empty database directories will be removed. However, it is possible for WiredTigerKVEngine::dropIdent() to not actually drop the ident right away. If WiredTiger returns EBUSY, the ident drop will be queued to be dropped later. If this happens, the database directory will not be removed since the directory is not yet empty by the time the callback is run.

A solution is to make KVEngine::dropIdent() optionally take a callback that will be run once the ident is actually dropped.