HPCC-8773 Add a control:closedown command to Roxie

For ease of debugging, but potentially useful in general, add a
control:closedown message to Roxie that will cause the Roxie node to close
down cleanly.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #3943 from richardkchapman/roxie-closedown

HPCC-8773 Add a control:closedown command to Roxie

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>