Add command to switch Gen2 servers

We are likely to continue wanting to connect to two (and possibly three) Gen2 servers: the operations server and the official simulator, plus perhaps an internal server for which we can control the UI via VNC – ie. from outside the four official control rooms.

So it'd be nice to switch between them via an MHS command. The crudest but clearly straightforward scheme would be to shell off a replacement, somehow committing suicide in the process. I'll try to re-instantiate the connection objects first.