The `rate` field `/api/campaigns/running/stats` returned was computed
based on the total time spent from the start of the campaign to the
current time. This meant that for large campaigns, if there were
pauses or slowdowns in between, the rate would be skewed heavily
making it useless to figure out the current send rate.
This commit introduces a realtime running rate counter in the campaign
manager that returns accurate (running) send rates for the last minute.
The `rate` field in the API now shows the live running rate and a
new `net_rate` field shows the rate from the beginning of the campaign.
0f6a0376da