Floating IP delete/attach/detach should be fully contained in transactions

[FelixMcFelix]

As of #4559, the above 3 operations use a fetched FloatingIp/ExternalIp to provide user-friendly error messages based on the current state of parent_id, before firing an UPDATE query conditioned on the correct parent_id field.

While this is not incorrect, this will provide a less user-friendly error message in the event of concurrent modification. We should move these datastore methods into a transaction and/or transaction_retry_wrapper to clean this up.

[bnaecker]

I'm pretty confident one can do this without an interactive transaction (with multiple DB roundtrips). Instead, the update_and_check() machinery could be used to attach or detach a floating IP with the desired conditions, such as parent_id IS NULL or similar.

[FelixMcFelix]

That looks like just the ticket, thanks! I hadn't yet come across it -- I'll see about working that in as a solution alongside #4630.