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>