Add MakeUnique (substitute for C++14 std::make_unique)

Summary:
Partial backport of core PR11043

Backported commits are 8617989, 3e09b39 and a357293

Depends on D2229

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2232