Implement simple moving average over work difficulty adjustement algorithm.

Summary: As per title. Hopefully this is much simpler than alternative, but more complex alternative do not perform suffisciently better to be worth it.

Test Plan:

make check

Added unit tests for it.

Reviewers: kyuupichan, Mengerian, #bitcoin_abc, schancel

Reviewed By: Mengerian, #bitcoin_abc, schancel

Subscribers: zanza321

Differential Revision: https://reviews.bitcoinabc.org/D601