Add 'MemoryKiller' Sidekiq middleware

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.
Add comments to the MemoryKiller middleware
Merge branch 'sidekiq_memory_killer' into 'master'

Sidekiq memory killer

This change adds an optional Sidekiq memory killer middleware that
will perform a graceful shutdown when the Sidekiq process is using
too much memory. When combined with a process supervision daemon
(such as Runit in omnibus-gitlab) this should provide Good Enough
handling of the Sidekiq memory leaks we are seeing.

The memory killer is off by default.

See merge request !1276