C++20 compatibility (#6697)

Summary:
Based on facebook/rocksdb#6648 (CLA Signed), but heavily modified / extended:

* Implicit capture of this via [=] deprecated in C++20, and [=,this] not standard before C++20 -> now using explicit capture lists
* Implicit copy operator deprecated in gcc 9 -> add explicit '= default' definition
* std::random_shuffle deprecated in C++17 and removed in C++20 -> migrated to a replacement in RocksDB random.h API
* Add the ability to build with different std version though -DCMAKE_CXX_STANDARD=11/14/17/20 on the cmake command line
* Minimal rebuild flag of MSVC is deprecated and is forbidden with /std:c++latest (C++20)
* Added MSVC 2019 C++11 & MSVC 2019 C++20 in AppVeyor
* Added GCC 9 C++11 & GCC9 C++20 in Travis
Pull Request resolved: facebook/rocksdb#6697

Test Plan: make check and CI

Reviewed By: cheng-chang

Differential Revision: D21020318

Pulled By: pdillinger

fbshipit-source-id: 12311be5dbd8675a0e2c817f7ec50fa11c18ab91
Signed-off-by: Changlong Chen <levisonchen@live.cn>

include/rocksdb/file_system.h

@@ -105,6 +105,8 @@ struct FileOptions : EnvOptions {
 
   FileOptions(const FileOptions& opts)
     : EnvOptions(opts), io_options(opts.io_options) {}
+
+  FileOptions& operator=(const FileOptions& opts) = default;
 };
 
 // A structure to pass back some debugging information from the FileSystem