Add support for transient federates

[erlingrj]

To handle coordinates startup and shutdown there needs to be a solid, known, base of federates. But we can also support transient federates, i.e. federates that are not part of the startup or shutdown procedure but can join at and leave at any time. We need to solve a couple of problems:

1. Acquiring the global start tag of the federation
2. Negotiating a local start tag when a transient boots and joins the federation.
3. Negotiating a local shutdown tag when a transients shutdowns and leaves the federation.
4. Negotiating a leaving tag, if a transient leaves the federation (without shutting down).
5. Negotiating a rejoining tag, if a left transient joins the federation again
6. Handling the scenario where a transient just leaves/shuts down without negotiating the tag at which is occurs.

I think several of these can be joined together as they are the same problem.

[edwardalee]

I'm not sure a leaving tag is necessary. A federate that leaves can be viewed as equivalent to one that stops producing outputs.