No queueing of high-frequency updates

[ashleydavies]

Pools will send messages as their contributions are updated, even if that is high frequency. Likely we will want to throttle this with an outbound messaging priority queue. This would need to understand that if a message is added to the queue with the same key as an older message, the older message should not be sent, and the new message should have the same priority as the older message. Until then, this is not suitable for high frequency updates (see limitations of MessagingService)