Remove HAVE_CELERY, HAVE_DJCELERY, HAVE_DJKOMBU.

Now required, these were optional long ago.
Backend and model cleanup.

Adds tests and documentation.
stats: refactor and clean up.

Stats records are now temporal, making it possible to look at historical
data with a minimum of queries.  Each row is stored with the Drink which
created it.

Stats messages are no longer defined by a protobuf schema, and so
can now use true dictionary types.