Added Memory Mapped Files, Tests and much more

[FlorianReimold]

• Performance improvements for FTP Downloads through memory-mapped file (Thanks to @bjuulp)
• Added googletest as submodule
• Added unit tests for fineftp-server. The tests require C++17 to compile and curl to be present in the PATH when executing them
• New CMake Options for Enabling / Disabling different components (See Readme.md)
• Binary downloads for Windows are now built with VS 2017 / v140 toolset
• Fixed a race condition on Windows that caused files not being fully flushed after fineftp-server reported the finished data upload (Thanks to @bjuulp)
• Reordered internal asio::strand implementation in order to prevent race conditions that haven't been detected, yet. (Thanks to @bjuulp)
• Fixed a vulnerability that enabled an attacker to access files above the root directory (reported by #52)
• Fixed many clang-tidy warnings
• The APPE (= append-to-file) command now creates a new file if it didn't exist already (this is the correct behavior according to RFC 959
• Updated the asio submodule to 1.28.2