Allow disabling WAL sync

This way Litestream can be used to only snapshot at regular intervals.

replica.go

@@ -228,6 +228,18 @@ func (r *Replica) syncWAL(ctx context.Context) (err error) {
 	}
 	defer rd.Close()
 
+	// If WAL sync is disabled, just advance our position.
+	if r.SyncInterval == 0 {
+		if _, err = io.Copy(io.Discard, rd); err != nil {
+			return err
+		}
+
+		r.mu.Lock()
+		r.pos = rd.Pos()
+		r.mu.Unlock()
+		return nil
+	}
+
 	// Copy shadow WAL to client write via io.Pipe().
 	pr, pw := io.Pipe()
 	defer func() { _ = pw.CloseWithError(err) }()
@@ -680,7 +692,12 @@ func (r *Replica) deleteWALSegmentsBeforeIndex(ctx context.Context, generation s
 
 // monitor runs in a separate goroutine and continuously replicates the DB.
 func (r *Replica) monitor(ctx context.Context) {
-	ticker := time.NewTicker(r.SyncInterval)
+	monitorInterval := r.SyncInterval
+	if monitorInterval == 0 {
+		monitorInterval = time.Second
+	}
+
+	ticker := time.NewTicker(monitorInterval)
 	defer ticker.Stop()
 
 	// Continuously check for new data to replicate.