This commit significantly improves the performance of campaign and group creation by changing database access to use transactions.
It should also make things more consistent with campaign creation. Specifically, this will ensure that the entire campaign gets created before emails start sending, while I anticipate this will fix#1643, #1080, (possibly) #1677, and #1552.
This builds on the work from @c-f in #1090 to fully add support for "timed" campaigns, in which the emails are spaced apart as opposed to all being sent at once.
Added the modified_date field to results so it's easy to keep track of the last results that were modified without having to parse every event. Updated the tests to reflect the changes.
Jordan Wright
Shuhei Kitagawa