tests: Fix stop of storage_service in cql_test_env

In stop() of storage_service, it unregisters the verb handler. In the
test, we stop messaging_service before storage_service. Fix it by
deferring stop of messaging_service.
Message-Id: <c71f7b5b46e475efe2fac4c1588460406f890176.1458086329.git.asias@scylladb.com>

tests/cql_test_env.cc

@@ -333,7 +333,6 @@ public:
 
             gms::get_gossiper().stop().get();
             gms::get_failure_detector().stop().get();
-            net::get_messaging_service().stop().get();
 
             _db->stop().get();
 
@@ -343,6 +342,8 @@ public:
 
             sstables::await_background_jobs_on_all_shards().get();
 
+            net::get_messaging_service().stop().get();
+
             bool old_active = true;
             assert(active.compare_exchange_strong(old_active, false));
         });