stream message has low priority than mailbox and queue

[maolin-cdzl]

sorry for poor english. in some really busy mlm server,stream message may block for quite long time. it's easy to test,keep send random mailbox/queue/stream message in a thread,recv message in another. recv thread never read any stream message. if stop sender thread,recv thread will read all stream message at last.

look into source code,maybe it is because zloop_t reader register order,router socket always be registed before any stream sockets.