Add support for the rdkafka 'stats_cb' config option in get_journal_client

this options allows to define a callback which will be called once every
'statistics.interval.ms' ms by rdkafka with a bunch of statistics (as a
json string).

See https://github.com/edenhill/librdkafka/blob/master/STATISTICS.md

This allows to define this callback as a string of the form:

"path.to.module:function"