streaming: Make stream_session::_handlers static

It is used to register the streaming message handler once. We don't need
it in every stream_session instance. The side effect is making
stream_session copyable, which is needed in get_all_stream_sessions.

streaming/stream_session.cc

@@ -86,6 +86,8 @@ void stream_session::init_messaging_service_handler() {
     });
 }
 
+distributed<stream_session::handler> stream_session::_handlers;
+
 future<> stream_session::start() {
     return _handlers.start().then([this] {
         return _handlers.invoke_on_all([this] (handler& h) {

streaming/stream_session.hh

@@ -123,7 +123,7 @@ private:
             return make_ready_future<>();
         }
     };
-    distributed<handler> _handlers;
+    static distributed<handler> _handlers;
     void init_messaging_service_handler();
     future<> start();
 public: