Upgrade to boost 1.72, current secp256k1, zeromq and Core consensus sources,
as well as removal of qrencode, png and zlib dependencies has been merged.
NuGet packages for secp256k1 have been uploaded, though I'm not intending to
update NuGet for zeromq.



Instead I'm planning to move to vcpkg, which is c++ oriented and includes
more current boost packages, and includes current zeromq packages. secp256k1
packaging continues to be a problem as the library is not versioned,
discouraging consistent packaging, so I'll end up building and maintaining a
package for it (only).



Once this is complete I will create vs2019 project files and remove
generation and testing of vs2013 (vc120/c++11 CTP).



There are Travis breaks on all macOS builds due to a Travis change affecting
boost installation. I'm going to resolve this next, and add builds for the
Travis big-endian and Windows platforms. Also on the build roadmap is
generation for mingw builds and VS Code projects for all three platforms.



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