Handle director loop

[nigoroll]

This ticket is to take a related, but different issue out of #3895

We do not currently handle director loops, which can cause an infinite recursion on resolution, inevitably leading to a stack overflow.

Two options are on the table right now

• when adding a src backend to a dst, ask src via a new callback if dst is reachable. The callback would iterate over all backends. If any matches dst, that would fail the add operation. Otherwise the callback would be called recursively.

• Limit the number of layers which director resolution can pass through.