The updater and on-demand requests kept separate copies of the SSH client
(sys.client and the transport's client) and synced them after each request.
That allowed a closed client to be reinstalled over a newer one and leaked
connections that were replaced without being closed.
The updater now dials, opens sessions and tears down timed-out connections
through the transport. The dial keeps the TCP keepalive and handshake
deadline, and an OnConnect callback handles the per-connection resets.
The transport is created under a lock and agentVersion is now atomic.
