Port binding hostID should be immutable when bound to a server

[mandre]

Taken from a discussion in #646, we identified a possible problem where a port could be bound to a different host than it's original binding:

Got it. I'd add in somewhere saying that the hostID from port doesn't affect the schedule of the server, as you can see in the test I made in the first comment, so I'm afraid to have a inconsistency state. So imagine:

1. Create a port with `hostID.ID = compute1`

2. Attach it to a server without any scheduler option to a host

3. The server spawn in compute2

4. The binding information for the port change to `compute2`

And in the port spec, we have compute1 still. Am I thinking correctly?

I believe you're right. But we have an identical problem if you pass the ID directly rather than a server ref. So perhaps we need to make hostID immutable?

Originally posted by @mandre in #646 (comment)

We'll need to conduct some tests and decide what the correct solution is in that case.