FileStore::_collection_move_rename: handle missing dst dir on replay

In case of a replay, a missing destination directory indicates that
the destination object and directory have been removed by a later
transaction.  Thus, we need to remove the src object and return
0.

Fixes: #6714
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>