lib, gapi: Next method of GAPI should generate first event

This puts the generation of the initial event into the Next method of
the GAPI. If it does not happen, then we will never get a graph. This is
important because this notifies the GAPI when we're actually ready to
try and generate a graph, rather than blocking on the Graph method if we
have a long compile for example.

This is also required for the etcd watch cleanup.

test/shell/libmgmt-change1.go

@@ -74,9 +74,6 @@ func (obj *MyGAPI) Graph() (*pgraph.Graph, error) {
 
 // Next returns nil errors every time there could be a new graph.
 func (obj *MyGAPI) Next() chan error {
-	if obj.data.NoWatch || obj.Interval <= 0 {
-		return nil
-	}
 	ch := make(chan error)
 	obj.wg.Add(1)
 	go func() {