Do not stop heartbeat even when manager is already stopped

* Heartbeat should never stop (discussion: https://github.com/mperham/sidekiq/commit/ba8c8a57b962489333ce2cc01bdcc0aa381572b8#commitcomment-5996837)
* Test that heartbeat stores data in Redis with ttl

Conflicts:
	lib/sidekiq/manager.rb
	test/test_manager.rb