Promote BIPs 123 and 2 Draft->Active (and implement them)

I plan to merge this on December 14th. If there are any hard objections to this change, please bring it up on the bitcoin-dev mailing list before then. Further reviews of the implementation are welcome in the meantime. Please refrain from requesting further changes to the BIPs themselves unless it is a blocker/show-stopper or trivial (not changing the meaning). https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-November/013331.html

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