Only start async mode executors when server is running

[bensheldon]

• Rails 6.1 has a Rails::Railtie.server block: Add a new server Railtie block: rails/rails#39953
• Webpacker checks whether Rails::Server is defined: https://github.com/rails/webpacker/blob/1aca8aaebd21a075d42e718d2b60dfba8e8d1b70/lib/webpacker/railtie.rb#L39-L48

Addresses trouble identified in #184

[bensheldon]

I'm thinking of introducing a new execution mode, async_server that will only run when Rails::Server is defined.

This matches how I'm currently running in my own applications and inside of the test app:

good_job/spec/test_app/config/environments/development.rb

Lines 64 to 70 in 91271d9

	if ENV['GOOD_JOB_EXECUTION_MODE']
	config.good_job.execution_mode = ENV['GOOD_JOB_EXECUTION_MODE'].to_sym
	elsif Rails.const_defined?("Server")
	config.good_job.execution_mode = :async
	elsif Rails.const_defined?("Console")
	config.good_job.execution_mode = :external
	end