fix(planstate): don't hold planLock while calling plan-changed listeners

[benhoyt]

Some listeners do quite a bit of stuff (check manager and log manager), and they're defined in packages external to the planstate package, so it's best to avoid holding our plan lock for the duration.

See spec OP040.

Also:

• PlanManager doesn't use state and runner, so remove them
• Rename planHandlers to changeListeners to match AddChangeListener