Implement mutex-guarded payment helper contract

[maraoz]

See "Remediation Approach 2: Mutexes" section here:
http://vessenes.com/more-ethereum-attacks-race-to-empty-is-the-real-deal/

[nventuro]

Our current ReentrancyGuard can be used to address these issues, and while more generic and with some limitations (guarded functions cannot call one another), I prefer it over a solution as specific as the proposed one.

[frangio]

This is also addressed by using PullPayment.