* Implement process stop faults via SIGSTOP / SIGCONT
* Implement RoundTripWorkload, which both sends messages, and confirms that they are received at least once.
* Allow Trogdor tasks to block until other Trogdor tasks are complete.
* Add CreateTopicsWorker, which can be a building block for a lot of tests.
* Simplify how TaskSpec subclasses in ducktape serialize themselves to JSON.
* Implement some fault injection tests in round_trip_workload_test.py
Author: Colin P. Mccabe <cmccabe@confluent.io>
Reviewers: Ismael Juma <ismael@juma.me.uk>, Rajini Sivaram <rajinisivaram@googlemail.com>
Closes#4323 from cmccabe/KAFKA-5849
Colin P. Mccabe