Improve CC's usage of Spring pre-loader to speed up test loading

[Gerg]

Update Spring preloader binstubs

• Should be functionally identical
• See rails/spring@02649d9

Improve Spring preloading

• Load the "init_block" of spec_helper pre-fork when using Spring
  • Previously the init_block was only used by Spork.
  • Spring would run it on every fork. This made tests load slower and
    reduced the benefit of using Spring, since less was being preloaded
    than could be.
• I blindly copied over the current init/run blocks, without
  investigating in any depth. It's possible that more can be moved into
  the "init" method, to further reduce test load times when using
  Spring. This could be a worthwhile follow-up, especially if/when Spork
  is removed.
• Extract out a SpecHelperHelper module so the "init_block" can be
  shared between spec_helper and Spring
• Enforce that SpecHelperHelper.init only runs once. That way it doesn't
  run post-fork, which is a minor performance improvement and means we
  don't need to worry about everything in there being idempotent.
• Add documentation to spec/README

Related to #4800

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