Make this more complicated, with a SoupMessageQueueItem to keep track of

Make this more complicated, with a SoupMessageQueueItem to keep track of

	* libsoup/soup-message-queue.c: Make this more complicated, with a
	SoupMessageQueueItem to keep track of the session's per-message
	state. (Part of the process of moving session-related state out of
	SoupMessagePrivate.)

	* libsoup/soup-session.c: Update to work in terms of
	SoupMessageQueueItem

	* libsoup/soup-session-async.c:
	* libsoup/soup-session-sync.c: use SoupMessageQueueItem (and get
	rid of SoupSessionAsyncQueueData and SoupSessionSyncAsyncData).

svn path=/trunk/; revision=1178