Avoid unnecessary lock in threaded producer
Avoid unnecessary lock in threaded producer

The JoinHandle is only accessed by the thread that holds the
ThreadedProducer, so it doesn't need to be wrapped in a RwLock. The
stop signal is already made threadsafe by use of an atomic bool.

thijsc/rust-rdkafka,fede1024/rust-rdkafka,thijsc/rust-rdkafka,fede1024/rust-rdkafka,fede1024/rust-rdkafka