Improve difficulty adjustment algorithm

[kseo]

References:

• select difficulty adjustment algorithm zcash/zcash#147
• select difficulty adjustment algorithm zcash/zcash#147 (comment)

[posgnu]

Excellent document about DigiShield
https://www.reddit.com/r/Digibyte/comments/213t7b/what_is_digishield_how_it_works_to_retarget/

[posgnu]

Quite good explanation about attack vector to KGW algorithm
zcash/zcash#147 (comment)

[posgnu]

Another good thread to refer
https://bitco.in/forum/threads/on-replacing-difficulty-algorithm-in-context-of-hard-forks.984/

[posgnu]

• bitcoinABC issue#75

[posgnu]

simulate bitcoin cash EDA
https://www.yours.org/content/simulating-the-bitcoin-cash-eda-e441804b863f

[posgnu]

A Description about MIDAS