Refactor sidekiq memory killer

What does this MR do and why?

This MR is extracting business logic that actually restarts the process in the separate handler (RestartSidekiqHandler)

It also:

• Simplifies the current logic
• Allows us to Introduce NullHandler in Introduce NullHandler for Sidekiq MemoryKiller, which will allow us to safely fine-tune soft limit and grace balloon period
• Simplifies specs - current specs are focused on implementation details and testing private methods, which makes refactoring and adding new logic very difficult
• Separates current monitoring phase (memory usage is fine, we are over the soft limit, we are over the hard limit), and lifecycle state (running, shutting down gracefully, killing sidekiq)

phase	description
phase_rss_within_the_range	Do nothing, RSS is within the range
phase_rss_above_soft_limit	Log and update metrics, only when the grace period is exceeded, handle high rss
phase_rss_above_hard_limit	Log and update metrics, handle high rss immediately

state	descripition
:running	Sidekiq process is running
:stop_fetching_new_jobs	handler sends SIGTSTP
:shutting_down	handler sends SIGTSTP
:killing_sidekiq	handler sends SIGKILL if process didn't shutdown gracefully

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Related to #368691 (closed)