Replace boost::unordered_map and boost::unordered_set with std:: variant

Summary:
Replaces boost implementation of hash map / hash set with variant
that is now in std c++ library. Addresses T70.

Test Plan: make check

Reviewers: kyuupichan, freetrader, sickpig, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D382