Refactor ichnaea.service.submit.tasks to merge wifi, cell paths.

This is not ready to land yet but I spent the past couple hours trying to figure out what's causing a spurious mysql error:

```
OperationalError: (OperationalError) (2003, "Can't connect to MySQL server on 'localhost' ((1040, u'Too many connections'))") None None
```

and am no closer than when I started, so I thought I'd put it up here for others to take a look at. This was some cleanup / factoring I wanted to land before attempting #238, since the existing paths are both under-factored as functions and heavily duplicated between the cell and wifi cases.