bindings/lua: take a reference on watch directory

Problem: lua segfaults when garbage collecting
kvsdir_t.

When a kvs_watch() callback receives a kvsdir_t
argument, it is only valid for the duration of the
callback.  When instantiating a directory as a Lua table
that persists after the callback, it looks like we need
to call kvsdir_incref() on the directory to avoid illegal
access and/or double free during garbage collection.

src/bindings/lua/flux-lua.c

@@ -1023,7 +1023,7 @@ static int kvswatch_cb_common (const char *key, kvsdir_t *dir,
     assert (lua_isuserdata (L, -1));
 
     if (dir) {
-        lua_push_kvsdir (L, dir);
+        lua_push_kvsdir_external (L, dir); // take a reference
         lua_pushnil (L);
     }
     else if (val) {