avoid race on event wireup

Send a copy of all event messages to any children so that they can start getting them immediately upon connection to the parent.  This ensures that events can be received when the 'live' module reports that all nodes are ready and rank 0 starts its shell.

Once duplicate events start to be received, send a `cmb.event-mute` message to the parent to close off the duplicate source of events.

Fixes issue #38