Introduces a conanfile.py (and a Conan recipe for RocksDB) to enable building the package with Conan, choosing more recent default versions of dependencies. It removes almost all of the CMake build files related to dependencies, and the configurations for Travis CI and GitLab CI. A new set of cross-platform build instructions are written in BUILD.md.
Includes example GitHub Actions workflow for each of Linux, macOS, Windows.
* Test on macos-12
We use the <concepts> library which was not added to Apple Clang until
version 13.1.6. The default Clang on macos-11 (the sometimes current
version of macos-latest) is 13.0.0, and the default Clang on macos-12 is
14.0.0.
Closes#4223.
* Builds Windows dependencies first.
* Builds ALL OSs in the last stage.
* Fix the MacOS builds.
* Windows dependency stages are allowed to fail so ALL configurations will
attempt to build. Windows builds will probably fail if dependencies fail
(caching may allow them to succeed), but they will at least be attempted.
* Remove broken AppVeyor config file, so it stops trying.
