Allow re-entrant actions to be processed

[stephencelis]

While unlikely in most TCA applications, it is possible for an action to enter the store when another action leaves the internal action buffer (e.g. on deinit), and sit in the buffer, unprocessed, till another action is received.

When buffered actions are processed, let's check the buffer one last time for any re-entrancy and, if it's not empty, kick off another send to process them.