External address queries should be idempotent.

[bnaecker]

External addresses are created during sagas. Multiple addresses are deallocated at once when an instance is destroyed. Neither of these is correctly idempotent. See the linked comment for more details, and suggested solutions. Will fill this out more as needed.

Originally posted by @bnaecker in #1298 (comment)

[bnaecker]

Added first part of this in #1458, which creates UUIDs as a separate saga action. Actual idempotency requires rewriting the allocation query.

[bnaecker]

Closed by #1493