Appveyor vs2019 static ssl fix

[lewk2]

This PR fixes a few things, and improves some others. In summary:

• fixes static OpenSSL linking for VS2015/VS2019 builds on AppVeyor (or anywhere anyone uses this with OpenSSL v1.0+)
• moves the scripted build into an _build folder (helps with .gitignore on workstations)
• places nuget packages (pthreads) into an _packages folder (helps with .gitignore on workstations)
• adds VS2019 building support to appveyor
• tunes up the options to reduce actions on VS2013/VS2015 and Debug builds for faster build times
• removes openssl from the artifact when statically linking
• moves unit testing to VS2019 (rather than VS2015) and only on release (unit test execution takes appreciable time)
• sets static openssl linking on VS2019 and VS2015, moves VS2013 to dynamic
• creates statically linked 'apps' in the VS2019 release build (for easy experimenting with single EXE)
• enables C++11 threading for VS2015 and VS2019 (leaves VS2013 on pthreads - C++11 build option fails on VS2013 anyway)

and finally... there it moves the actual build scripting work out of appveyor, into a dedicated (and much expanded) powershell script in the 'scripts' folder.

This new build script allows:

• easier developer flag control without opening cmake (uses powershell auto-completion)
• quicker debugging of build issues on local workstations without endless commits to test build script in appveyor itself
• opens the door to other CI systems just calling this script (GitHub actions?)
• helps people compiling locally to install extra tools (cmake, pthreads) and provides feedback
• allows double-click building with handy wrap-around 'build-windows.bat' that calls the powershell

While this is a decent change to the build process, it does not invalidate the current windows building instructions - although they could potentially be reviewed after merging to see if there are further optimizations to those instructions. However, those instructions are very informative for people wishing to step away from the scripted build - so at most a 'quick start' section referring to this BAT-file based execution could be considered.

Finally, quite understandably, total time to run these builds is important - so even though more combinations have been added within this PR, through careful allocation of flags and skipping 2013 debug builds, the total build time is reduced by ~33% (~24 mins down from ~34mins)

[ethouris]

Please consider the comments above and answer questions if possible.

[maxsharabayko]

moves unit testing to VS2019 (rather than VS2015) and only on release (unit test execution takes appreciable time)

This will probably hide this strange behavior.
I would also suggest run unit tests for at least one of the builds using pthread to ensure it still works.

[maxsharabayko]

Merging to improve AppVeyor build time.