[reconfigurator] Abandon orphaned sagas during execution

[karencfv]

Implements a way to abandon orphaned sagas during blueprint execution.

To be conservative, we only abandon sagas whose current_sec is an expunged (and ready-for-cleanup) Nexus zone in the target blueprint whose generation is strictly older than the oldest in-service Nexus generation. Any Nexus that isn't in the target blueprint is left untouched, since we can't confirm its state.

Closes: #10911

This should be merged AFTER release 22: I would prefer it if this commit spent some time on the dogfood rack before shipping it to customers.