Better debugging for memory killer middleware

This adds more info to the warning messages output by `MemoryKiller`.

Previously only the PID was showed, making it difficult to debug issues like https://gitlab.com/gitlab-org/gitlab-ce/issues/19124

This adds the worker class and job ID to the log messages.

See merge request !4936
(cherry picked from commit 3659992c)

This makes the memory killer behave more like 'sidekiqctl stop'.

This makes the memory growth-triggered Sidekiq restarts more reliable by
reducing the chance that Sidekiq ends up in a state where it is not
accepting new jobs but also not shutting down: SIGKILL is more likely to
work than SIGTERM.

It looks like SIGTERM may not be enough to shut down a Sidekiq process
when its RSS has gotten too big. This change will allow us to experiment
with sending SIGKILL instead of SIGTERM to Sidekiq processes on
gitlab.com.

When enabled, this middleware allows Sidekiq to detect that its RSS has
exceeded a maximum value, triggering a graceful shutdown. This
middleware should be combined with external process supervision that
will restart Sidekiq after the graceful shutdown, such as Runit.