[op/rename]: Cycles

[liaud]

Without resorting to locks, although the kernel prevents a local rename to create a cycle, there is no check mechanism in case of concurrency.

With the current state of ElmerFS, it is not obvious if we can't prevent that without resorting to a tree crdt on the client side or in AntidoteDB directly.

[dan-da]

In case it is of any interest, I made a tree crdt in rust a couple years back that avoids cycles. I used it in sn_fs another experimental distributed filesystem. To be clear, it's just prototype code, not efficient.