log mem/cpu stats for jobs as well

This logs the STAT line in the same format as for web requests, though
omitting the @@last_sample at the end.

Rather than implementing it directly in worker.rb, I pulled in the new
Lifecycle callback code that delayed_job added after we forked, and
added the logging as an around filter in our canvas initializer.
Eventually I'd like to pull the other canvas-specific code into
lifecycle callbacks as well, making delayed_job more generic and
extractable.

test plan: run jobs, and verify that they still complete successfully,
and log a STAT line at the end with information on cpu and memory usage.

Change-Id: I18e17b286f5df8aa3c9774f0ac92841f4b92e193
Reviewed-on: https://gerrit.instructure.com/30037
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>