core: Parse the tags list sooner, and use it for multiple function

core: Parse the tags list sooner, and use it for multiple function

 - Parse the tags list using strv_split_newlines() which remove any
   unnecessary empty string at the end of the strv.
 - Use this parsed list for manager_process_barrier_fd() and every call
   to manager_invoke_notify_message().
 - This also allow to simplify the manager_process_barrier_fd() function.