Simple latched tcp topics

[peci1]

The simpler version. Still don't know which one to choose... This one needs a bit more documentation.

[xqms]

I think we still have to solve the situation that the receiver starts after the sender, but there are no new messages. In this situation, connect() is not called, and latched messages are not sent.

I would propose starting a "connection" timer, that tries to connect (if there is no connection).
To reliably detect connection loss (e.g. the receiver was restarted), we should also probably set the SO_KEEPALIVE socket option.

This whole machinery should only be active if we actually have latched topics.