Sidekiq currently overrides the trap. This change
defers the definition of the signal handler until
after sidekiq's setup code.

This way, we get to keep our signal handler, and
stackprof can be invoked by sending a SIGUSR2 to
sidekiq processes.

Because sidekiq-cluster forwards SIGUSR2 signals,
we can also send the signal to the sidekiq-cluster
process.