Move general tasks from `ichnaea.tasks` to `ichnaea.data.tasks`.
Rename `heka_logging` to `logging`.

We are moving away from heka-py and towards individual clients, like we
already did for the statsd client. Let the module name reflect this.
Move stats_client into general logging module.
For celery tests use Redis instead of MySQL to better match production.
Add some comments about command line entrypoints.
Rename config module to app_config.

This distinguishes it a bit more clearly from the gunicorn and async config
modules.
Note `-Ofair` argument being used for celery worker.
Pull out some constant values into a new constants module.
Move normalization/validation logic into new `data.validation` module.
Move validation related constants into validation module.
Reorder tasks/tests inside data subpackage alphabetically.

Also use a cell/wifi suffix instead of prefix for the location_update tasks.
Avoid some code duplication in stats related content tasks.