This adds support for the new TestNetv4 and ScaleNet chains started by BCHN to replace the current TestNetv3 chain.
As a precursor, the first commit changes the November 2019 and May 2020 hard fork activation checks to use the height when the forks were historically activated instead of using the MTP calculation. Fork heights for MainNet, TestNetv3, and Regtest were compared against both BU and BCHN source. I did notice a 1 block discrepancy between BU and BCHN on TestNetv3 for the May 2020 hardfork. I arbitrarily used the value from BCHN in this MR, I'm not certain which is actually correct.
As part of this, all relevant documentation that I was aware of was updated.
I have test nodes up and running on both chains for the past several days and even mined a few blocks with the client on each chain.
Test nodes all running under Ubuntu 20.04, headless. I have not tested the Qt client.