zb: Fix NoneBefore returns in MessageStream
Because MessageStream no longer receives updates on every message, its internal tracking of next_seq is no longer valid. This adds a field to Connection that tracks the last message that has been completely broadcasted to all MsgBroadcasters and has MessageStream use that field instead.
This should fix the ordered-stream
deadlocks introduced in !628 (merged) even without the unblocking changes in 0.1.3.