Log session length in disconnection message

[uuumisha]

imho: It's convenient to know how long the session lasted.

[djmdjm]

If you turn on LogLevel=verbose this information is already available on the client

Connection to blah closed.
Transferred: sent 4228, received 4588 bytes, in 2.0 seconds
Bytes per second: sent 2114.0, received 2294.0