Add 'requiring': like 'ensuring' but with 'require' semantics. Fixes #2798

Improves upon

    require(d > Duration.Zero, "failure-detector.heartbeat-interval must be > 0"); d

by allowing the construction

    d requiring (_ > Duration.Zero, "failure-detector.heartbeat-interval must be > 0")

Uses `require` internally so the semantics should be the same.

Questions:
- name?
- location - is akka.util.Helpers.Requiring, OK?
- unit tests needed?