Hook up new presence to apiserver and status.

[howbazaar]

This branch adds two new logical pieces, and hooks up a third.

Features are added to controller config, and when config is updated, a pubsub message is sent for controller.ConfigChanged.

Added a new apiserver struct for shared server data. This is for server attributes that don't change from connection to connection, and need to be threaded through to the facade context. The state pool, presence, and now the central hub are all part of this context. This shared context also knows what controller feature flags have been set.

The last piece is to hook up the status code to the new presence. This means setting the ModelPresence when calling status. There was a logic bug in the controller connections in that all were considered controller connections which meant that AgentStatus always showed unknown. The controller connection flag is supposed to be for non-controller models for controller machine connections. This has now been updated.

QA steps

juju bootstrap lxd test
juju enable-ha
watch -c juju status -m controller
juju ssh -m controller 2
# in there, do this
sudo service jujud-machine-2 stop
# observe the slow status updating showing machine 2 down
# in another terminal
juju controller-config features=[new-presence]
# now stop and start machine 2 and observe faster status.

Documentation changes

This isn't a user observable change, although we may want to document the controller feature flags options.

[wallyworld]

Reviewed live, lgtm with apiserver closing context and todo for config versioning

[howbazaar]

$$merge$$