Zk kafka plugins

Add a zookeeper config plugin to load json string config from zookeeper;

Add a kafka log receiver plugin, the plugin can only send messages less then 900 Bytes each time. The plugin is not actually practical at the moment, may someone help to figure out a proper way to code with LibKafka.

Yours,
Imcom
We've made kind of philosophical decision to never `use` namespaces unless we reallllly had to (ie: `apache::thrift::concurrency` is way too long, so we `used` that.)

Can you remove these two `using` lines?