materialize: add missing log message for when evaluating loads starts

Right now there is no immediate log message when evaluating loads starts, but
there is supposed to be. This adds that missing log.

materialize-boilerplate/logging.go

@@ -391,6 +391,7 @@ func (l *BindingEvents) do(fn func()) {
 // destination database.
 func (l *BindingEvents) StartedEvaluatingLoads() {
 	l.do(func() {
+		l.log(log.Fields{"round": l.round}, "started evaluating loads")
 		l.evaluatingLoadsStart = time.Now()
 
 		l.stopLogger = make(chan struct{})