Worker: completely flush upstream buffer at startup

Completely flush the upstream buffer at startup, before attempting to
fetch a new task. These updates could impact any task on the Manager side,
and first flushing the buffer before appending new updates also seems
like a good idea.

cmd/flamenco-worker/main.go

@@ -94,7 +94,8 @@ func main() {
 
 	timeService := clock.New()
 	buffer = upstreamBufferOrDie(client, timeService)
-	go buffer.Flush(workerCtx) // Immediately try to flush any updates.
+	// Flush any updates before actually starting the Worker.
+	buffer.Flush(workerCtx)
 
 	cliRunner := worker.NewCLIRunner()
 	listener = worker.NewListener(client, buffer)