Merge with bitcoin 0.18

[cornelius]

Bitcoin Core has branched 0.18 now: https://github.com/bitcoin/bitcoin/tree/0.18. It aims at a 0.18.0 release on Apr 3 2019 (bitcoin/bitcoin#14438).

[cornelius]

There is a pull request to make clonemachine ready for 0.18: dtr-org/unit-e-clonemachine#35