Alert sharding produces multiple groups

[sinkingpoint]

The way we send alerts at the moment is that for each alert, we generate an "authoritative" node which is responsible for sending the alert. This means that alerts that end up in the same group can be on different authoritative nodes resulting in multiple groups (one for each node) being fired.

To fix this, we'll need to make that sharding group aware - using the group key, rather than on a per alert basis