cephfs, tools/cephfs_mirror: migrate from boost::variant to std::variant

cephfs, tools/cephfs_mirror: migrate from boost::variant to std::variant

Replacing boost::variant with std::variant throughout the cephfs related
codebase. This change is part of our ongoing effort to reduce
third-party dependencies by leveraging C++ standard library alternatives
where possible.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>