Workaround container self-stop race

[MggMuggins]

If an instance self-stops while statusCode() is waiting for getLxcState() to finish, statusCode() may return a stale instance state.

This PR is a workaround for the use-case in #13453 and significantly reduces the likelihood that statusCode returns a stale status.

In an ideal world, LXD would maintain a canonical cluster-wide view of instance state. This would allow making race-free decisions based on whether an instance is running or not. For example:

• Project CPU/RAM limits could be enforced at instance start instead of at instance creation
• Volumes with content-type block could be attached to more than one instance without security.shared; instance start could fail if another instance with any shared block volumes is already running.