Major Description
So it occurs to me that adding ports before balancing in is a good idea so clients can connect straight away, however deleting as part of the same operation will cause clients to fail during the rebalance.
To be correct we create/update pre rebalance, and delete post rebalance.
Phew. Jumping the gun a bit
These are cleaned up when they cease to be members, e.g. are destroyed after the rebalance, so we're safe.