And use it to allow overwrites of old address assignments in the conn25 client.
The magic and transit address pools from which the addresses come are limited
resources and we want to reuse them. This commit is a small part of that bigger
need.
We expect to follow soon:
* Extending expiry if assignments are still in use.
* Returning expired addresses back to the pools so they can be reallocated.
Updates tailscale/corp#39975
Signed-off-by: Fran Bull <fran@tailscale.com>
d7916d4369