Poll module: time checking

[ashuping]

The poll module currently only updates a poll when a new vote is cast. This means that, when a poll reaches its open or close time threshold, rendered messages do not update to reflect the new status until somebody tries to cast a vote. A system needs to be implemented whereby the module automatically updates all polls that have opened or closed at a regular time interval (for example, every minute).

In order for this to work without being extremely resource-intensive, the scope of polls to update should be restricted to polls which have closed since the last update and polls which have opened since the last update (open polls will already be updated whenever somebody votes on them, so we only need to check if the time thresholds for opening/closing have passed).