librbd: s/boost::variant/std::variant/

librbd: s/boost::variant/std::variant/

boost::variant explicitly prevent it from being compared other types
by marking the return type of, for instance operator==(const T&) as
"void", if T is not identical to the variant type. so address this,
let's use std::variant<> instead.

more standard compliant and simpler this way.

because of https://cplusplus.github.io/LWG/issue3052, libstdc++
only specializes for std::variant<> for std::visit(), while libc++
allows the derived types of std::variant<> to be "visited", hence
we add an adaptor for SnapshotNamespace.

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