examples/asynchronous_processing: use updated Tokio API (#110)
examples/asynchronous_processing: use updated Tokio API (#110)

The tokio::executor::{current_thread, thread_pool} exports have been
deprecated since Tokio v1.8. Use the newer runtime APIs instead.
There is no functionality change; this just makes the asynchronous_processing
example clearer for folks who are new to the Tokio ecosystem.
fede1024/rust-rdkafka,thijsc/rust-rdkafka,fede1024/rust-rdkafka,fede1024/rust-rdkafka,thijsc/rust-rdkafka