Normalize the get_journal_client() function

make it similar to other get_xxx factory functions we have:

• make it take a cls first argument (only supports "kafka" for now),
• take **kwargs as arguments to be passed to the class constructor.

This comes with the following changes in configuration:

• the config entry for the journal client is now "journal_client" (instead of "journal"),
• the journal_client config dict must have a "cls" field (only "kafka" being supported for now).

Compatibility should be ensured.