Move SidekiqPlugin#job_keys to a class constant
Move SidekiqPlugin#job_keys to a class constant

The value of the method doesn't change and is used throughout all
Sidekiq jobs. Not having to define it for every job saves us some
memory.

appsignal/appsignal,appsignal/appsignal,appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby,matrinox/appsignal-ruby,matrinox/appsignal-ruby,appsignal/appsignal-ruby,matrinox/appsignal-ruby