Slowly queue outgoing notifications

[mjl-]

We currently queue all outgoing messages as quickly as we can, once every 10/15 minutes when the tree moves forward. If there are new modules/versions that many users subscribe too, we'll be queuing lots of messages in a short time. The receiving mail esrvers (e.g. many users at gmail.com) may not like that. And it may not be great for our load either.

We can either just queue them slowly. Or we can use the SMTP futurerelease extension to let the mail server send messages slowly.
By managing this ourselves, we can get better behaviour. Such as combining another set of updates (that come in soon after) in the same message.