seastar::apply() is deprecated in recent versions of seastar in favor of std::apply(), so stop including its header. Calls to unqualified apply(..., std::tuple<>) are resolved to std::apply() by argument dependent lookup, so no changes to call sites are necessary. This avoids a huge number of deprecation warnings with latest seastar. Message-Id: <20200526090552.1969633-1-avi@scylladb.com>