When enable daemon memory killer, old memory killer will be disabled

The daemon memory killer(Gitlab::SidekiqDaemon::MemoryKiller)
 - is to replace existing Gitlab::SidekiqMiddleware::MemoryKiller
 - allows Sidekiq memory balloon reasonably, within given time limit
 - reduces monitoring overhead(when there are many small jobs)
 - monitors Sidekiq memory timely(for long running jobs)
 - respects Worker sidekiq_options `memory_killer_memory_growth_kb`
 - respects Worker sidekiq_options `max_memory_killer_memory_growth_kb`

 Update document on Sidekiq memory killer ENV variables