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.
* Refactoring servers to support custom workers and graceful shutdown.
* Refactoring workers to support custom mailers.
* Refactoring mailer to be an interface, with proper instances instead of a single global instance
* Cleaning up a few things. Locking maillogs for campaigns set to launch immediately to prevent a race condition.
* Cleaning up API middleware to be simpler
* Moving template parameters to separate struct
* Changed LoadConfig to return config object
* Cleaned up some error handling, removing uninitialized global error in models package
* Changed static file serving to use the unindexed package
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.
