remove unneeded (locking ...) around SSE channels

The current code locks channels in order to preserve ordering when sending SSE events. This patch removes these locks and instead uses `mk-data` to construct a single large message that contains all data. There really should never be a time when we'd need to lock a channel. 

Eventually this could possibly be improved by using a transducer in the channel that contains a call to `flat-map`. This would allow for vector messages to be spliced in, while preserving ordering. But for now this patch should be good enough.