Make unbound keys expire with a different TTL in Vault

[gnarea]

(Bound key: A session key that can only be used with a specific recipient; unbound key: A node key or an initial session key)

Bound keys should expire after whatever TTL is set in Vault, but unbound keys should normally be kept around for much longer, so that default TTL should be overridden when unbound keys are created (which requires making a second API call unfortunately).