transport: ignore errors during connection shutdown

If the other end of the connection has already disconnected the shutdown
will fail with ENOTCONN. The resulting exception is going to propagate
through the continuation chain that is supposed to shut the cql server
down preventing it from properly waiting for all outstanding
continuations.

The solution is to just ignore any errors that shutdown() may return.

Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>

transport/server.cc

@@ -544,9 +544,12 @@ future<> cql_server::connection::process()
 
 future<> cql_server::connection::shutdown()
 {
-    return _fd.shutdown_input().then([this] {
+    return make_ready_future<>().then([this] {
+        return _fd.shutdown_input();
+    }).then_wrapped([this] (auto&& f) {
+        f.ignore_ready_future();
         return _fd.shutdown_output();
-    });
+    }).handle_exception([] (auto) {});
 }
 
 future<> cql_server::connection::process_request() {