• NOTE: this is a recommended update for anyone running v1.15.21.
• Added a timeout to h2 worker cleanup to exit latest after 5 seconds of
  waiting on idle workers to terminate. This happens after all connections
  have been processed. a WARNING is logged in case workers lagged behind.
  This may happen in busy situations or depending on configurations that
  have slow responses.
  This fixes #212, many thanks to @famzah for his testing and feedback!
• H2 sessions normally log a warning when the connection is torn down
  without them being able to send a last GOAWAY frame. This logging
  happens at DEBUG level during the stopping of a child process.
• configure now checks for apache2ctl if apachectl was not found.
  This fixes #218.
• configure now longer errors when no apachectl was found but warns
  that the test suite will not work.
• The test suite now clears Apache's error log on start and checks after
  the run if unexpected errors or warnings were logged.
• Added top level make target docker-test to run all defined test images.
  This closes #217.