transport::cql_server::connection::process(): Consume the last future

When process() terminated _ready_to_respond contains the future with the
state of the last I/O operation of this connection. Consume it at the
end of process().

Fixes issue #44

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>

transport/server.cc

@@ -149,7 +149,8 @@ class cql_server::connection {
         , _client_state(service::client_state::for_external_calls())
     { }
     future<> process() {
-        return do_until([this] { return _read_buf.eof(); }, [this] { return process_request(); });
+        return do_until([this] { return _read_buf.eof(); }, [this] { return process_request(); })
+        .finally([this] { return std::move(_ready_to_respond); });
     }
     future<> process_request();
 private: