generic_server: use async function in for_each_gently()

In the following patch, we will add a method to update service levels
parameters for each cql connections.
To support this, this patch allows to pass async function as a parameter
to `for_each_gently()` method.

generic_server.cc

@@ -40,13 +40,12 @@ connection::~connection()
     _server.maybe_stop();
 }
 
-future<> server::for_each_gently(noncopyable_function<void(connection&)> fn) {
+future<> server::for_each_gently(noncopyable_function<future<>(connection&)> fn) {
     _gentle_iterators.emplace_front(*this);
     std::list<gentle_iterator>::iterator gi = _gentle_iterators.begin();
     return seastar::do_until([ gi ] { return gi->iter == gi->end; },
         [ gi, fn = std::move(fn) ] {
-            fn(*(gi->iter++));
-            return make_ready_future<>();
+            return fn(*(gi->iter++));
         }
     ).finally([ this, gi ] { _gentle_iterators.erase(gi); });
 }

generic_server.hh

@@ -119,7 +119,7 @@ protected:
 
     virtual future<> unadvertise_connection(shared_ptr<connection> conn);
 
-    future<> for_each_gently(noncopyable_function<void(connection&)>);
+    future<> for_each_gently(noncopyable_function<future<>(connection&)>);
 
     void maybe_stop();
 };

transport/server.cc

@@ -2022,9 +2022,10 @@ void cql_server::response::write(const cql3::prepared_metadata& m, uint8_t versi
 
 future<utils::chunked_vector<client_data>> cql_server::get_client_data() {
     utils::chunked_vector<client_data> ret;
-    co_await for_each_gently([&ret] (const generic_server::connection& c) {
+    co_await for_each_gently([&ret] (const generic_server::connection& c) -> future<> {
         const connection& conn = dynamic_cast<const connection&>(c);
         ret.emplace_back(conn.make_client_data());
+        return make_ready_future<>();
     });
     co_return ret;
 }