Use c++14 generic std::rbegin() and std::rend() instead of class methods

Summary:
This makes the code more consistent

Depends on D2462

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2464