node: improve node state and lifecycle management

[dshulyak]

• working with node requires access to private fields (signer and wait group)
• additionally there is a mess with lifecycle, it accepts context and Start blocks on it, but in order to wait you have to call Cleanup and then wait on private error group

/* Create or load miner identity */
				if app.edSgn, err = app.LoadOrCreateEdSigner(); err != nil {
					return fmt.Errorf("could not retrieve identity: %w", err)
				}

				if err := app.LoadCheckpoint(ctx); err != nil {
					return err
				}

				if err := app.Initialize(); err != nil {
					return err
				}
				// This blocks until the context is finished or until an error is produced
				err = app.Start(ctx)

				cleanupCtx, cleanupCancel := context.WithTimeout(context.Background(), 30*time.Second)
				defer cleanupCancel()
				done := make(chan struct{}, 1)
				// FIXME: per https://github.com/spacemeshos/go-spacemesh/issues/3830
				go func() {
					app.Cleanup(cleanupCtx)
					_ = app.eg.Wait()
					close(done)
				}()