Use SIGKILL only as a last resort in Sidekiq MemoryKiller

A recent customer issue caused us to configure Sidekiq MemoryKiller from SIGKILL to SIGTERM to prevent losing jobs upon termination. c291ff9c changed the default to SIGKILL. Should we try SIGTERM first, and then reserve SIGKILL for cases where the job doesn't get killed after some time (e.g. 10 seconds)?

/cc: @dblessing, @jacobvosmaer, @yorickpeterse