Support for more compilers

During last week we have added support for compiling Talleo codebase with more recent compilers. We have tested with GCC 6-10 and Clang 6-11.

Some warnings were silenced if fixing the underlying issue would have required breaking compatibility with older compilers or extensive modifications in third-party libraries.

Smaller issues however were fixed also in included sources of third-party libraries, including RocksDB and gtest.

We will continue testing the build with different compiler and cmake versions as some failures were triggered only when specific build options were used with specific cmake version.

We added GitHub Actions workflow for testing on Ubuntu 18.04 and 20.04 and updated and enabled the existing AppVeyor configuration to test build with Visual Studio 2017.9.