Add futures to pipeline that copy the real redis's behaviour

Based on this pull request:

  https://github.com/causes/mock_redis/pull/62

with some style fixes applied:

  - improved commit message formatting
  - removed `puts` debug statements
  - remove some unnecessary blank lines
  - converted double quotes to singles
  - squashed both commits in the pull request into one
  - tidied up some whitespace (mostly alignment)
  - dropped unnecessary explicit `(:each)` from `before` blocks
  - dropped "should" from `it` descriptions

Signed-off-by: Greg Hurrell <greg@causes.com>
Change-Id: If2b62f830efca103dd29b7f4a863954a65acd342
Reviewed-on: https://gerrit.causes.com/26222
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Kim Douglas Clarence Mason <kim.mason@causes.com>