Implement NULLDUMMY softfork (BIP147) #8636
This is an implementation of BIP147, which could be found at https://github.com/bitcoin/bips/blob/36496946860d71d4460437572e6c4c780b125cad/bip-0147.mediawiki
I propose that reviewers should explicitly acknowledge that this pull request is a correct implementation of a specified version of BIP147. So they should review the relevant codes, for example, in interpreter.cpp.
This is needed because in #8533 we just focused on the PR, but not the consensus codes being activated. NULLDUMMY should be reasonably simple for everything to do more careful review.
For example, a reviewer may use: utACK 482f852, ACK BIP147 3649694