There is a gcc9 bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90415
that makes it impossible to pass std::any through a seastar::future<T>.
Fortunately, there is only one user of seastar::future<std::any> in
Scylla and it is not performance-critical. This patch avoids the gcc9
bug by using seastar::future<std::unique_ptr<std::any>>.