RUBY-1080 Fix for Delayed Job sampler not loading properly
RUBY-1080 Fixing startup sequence to call after_fork correctly

Because of our changes in startup, we were inadvertently trompling the
ActiveRecord backend's after_fork hook in DJ. This problem was latent before,
but exposed by the change to startup the agent more aggressively in 3.6.1.

Additionally this removes the alias_method calls against instances of the DJ
backend, since that doesn't even make sense in how DJ actually exercises the
backend code. See the commit at https://github.com/collectiveidea/delayed_job/commit/855f1ba9a18c2f946aa82314731ed89669d41a66