Convert LUConnectConsole to concurrency

For ConnectConsole we just need to lock the instance we're connecting
to. We make a few rpcs to its primary node, but node daemons can now
handle multiple queries and nodes cannot be removed till they have
instances on them anyway. Note that since we return the ssh command, and
that's executed outside of the ganeti daemon, without any locks held,
the instance can then be subject to operations while we're connected to
it, but that was the previous behavior as well.

Reviewed-by: iustinp