Import keepalive feature and encrypted client key from Incus

https://github.com/lxc/incus/pull/295 => keepalive feature: keep connection alive for a certain time avoiding the setup cost
https://github.com/lxc/incus/pull/468 => support SSH encrypted key for authentication: allow decrypting the client.key once when combined with the keepalive feature
https://github.com/lxc/incus/pull/481 => followup on PR 468