2.9.0: config.rd incorrectly caches group_id when using default

[nic-snoek-zepto]

Since version 2.9.0:

• When starting multiple consumers in the same process (in different threads)
• And relying on the default group_id (group_id_prefix + classname.kebabcase)

config.rb caches the group_id and does not recalculate for different consumer class.

Workaround: Do not rely on default group_id, but set group_id explicitly on each consumer class.