Remove KegbotCache callers.

The generational cache has been dormant for a while.
charts: Render error string correctly.
backend: Include event & stats generation in transactions.

Though these are derived, it doesn't make much sense to
successfully record a change without them.

Issue #264.
Fix indent.
Downgrade Celery and kombu.

See issue #264
plugin: Don't decode as kbjson.

Immediate purpose is to disable automatic ISO8601
conversion, which may not be appropriate for plugin data.
stats: Rebuild only when necessary.