Message stores can become corrupted on acceptor/initiator shutdown

Since MessageStore implementations aren't thread safe, all operations on them should be performed from the session thread only. However, all 4 acceptor and initiator implementations perform shutdown (and thus call Session.unregisterSessions) in the calling thread. Since that calls MessageStore.close(), it can easily lead to store corruption if, for example, FileStore.close() gets called while the session thread is in the middle of persisting a Logout response.