Bug: Management RPC keyset rotation malforms keyset timestamps

[orangeshyguy21]

Someone recently reported this issue in an sqlite mint upon keyset rotation.
I confirmed the behavior on a test mint on 0.18.1 using the rpc method to rotate keysets.

Upon rotation the outgoing keyset has it's timestamp fields (valid_from, valid_to, first_seen) nullified.

[orangeshyguy21]

a subsequent rotation did not nullify the outgoing keyset timestamps

If the mint process is restarted before the rotation the bug is reproducible