migrations: don't start model workers while a model is importing

[babbageclunk]

Fixes https://bugs.launchpad.net/juju/+bug/1646310

(Recreated for develop branch from #6678.)

Starting the migration master while the model was being imported back
into a controller that it had previously migrated out of caused it to
uninstall itself from the engine (thinking that it was finished
processing the old migration), which blocked any workers that depended
on ifNotMigrating. To fix this we wait until the model MigrationMode
has changed to MigrationModeNone - this means the watcher needs to
signal on any changes to models, rather than just life.

Add a global flag to the collectionWatcher config - this lets the watcher return
ids for any model, rather than just the current one.

The apiserver code to remove state objects from the pool still needs to
watch life specifically. If the model becomes Dead and is then
removed within the event coalescence time then the collection watcher
won't report any event, but a lifecycle watcher will.

QA steps:

• bootstrap two controllers A and B
• create a model m in A with an application deployed
• migrate it to B
• migrate it back to A
• check the debug log has no dependency engine messages saying "fortress operation aborted" - these show the workers that are prevented from running because the migration master has exited
• migrate it back to B - this would have failed before this fix, because the previous migration left the model workers in a bad state

[babbageclunk]

Doesn't need rereview - no difference between this and #6678.

[babbageclunk]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9863

[babbageclunk]

Failed to bootstrap on lxd. ):

$$retry$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9864

[babbageclunk]

Windows tests failed for something to do with Mongo:
wsarecv: An existing connection was forcibly closed by the remote host.

$$retry$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[babbageclunk]

Yay!