Allow setting replica snapshot interval offset

If using S3 replicas the extra LIST call on startup may be
expensive in scale if a lot of Litestreams are started in quick
succession.

Allowing configuring the snapshot offset externally allows the
caller to spread around the snapshots without relying on access to
the remote replica making restarts no-op.

replica.go

@@ -55,6 +55,9 @@ type Replica struct {
 	// Frequency to create new snapshots.
 	SnapshotInterval time.Duration
 
+	// Duration added to snapshot interval before the first one. Skips checking replica.
+	SnapshotOffset time.Duration
+
 	// Time to keep snapshots and related WAL files.
 	// Database is snapshotted after interval, if needed, and older WAL files are discarded.
 	Retention time.Duration
@@ -738,26 +741,31 @@ func (r *Replica) snapshotter(ctx context.Context) {
 	}
 
 	logger := r.Logger()
-	if pos, err := r.db.Pos(); err != nil {
-		logger.Error("snapshotter cannot determine generation", "error", err)
-	} else if !pos.IsZero() {
-		if snapshot, err := r.maxSnapshot(ctx, pos.Generation); err != nil {
-			logger.Error("snapshotter cannot determine latest snapshot", "error", err)
-		} else if snapshot != nil {
-			nextSnapshot := r.SnapshotInterval - time.Since(snapshot.CreatedAt)
-			if nextSnapshot < 0 {
-				nextSnapshot = 0
+	if r.SnapshotOffset == 0 {
+		if pos, err := r.db.Pos(); err != nil {
+			logger.Error("snapshotter cannot determine generation", "error", err)
+		} else if !pos.IsZero() {
+			if snapshot, err := r.maxSnapshot(ctx, pos.Generation); err != nil {
+				logger.Error("snapshotter cannot determine latest snapshot", "error", err)
+			} else if snapshot != nil {
+				r.SnapshotOffset = r.SnapshotInterval - time.Since(snapshot.CreatedAt)
+				// ensure we will snapshot immediately if zero than less
+				if r.SnapshotOffset < 1 {
+					r.SnapshotOffset = 1
+				}
 			}
+		}
+	}
 
-			logger.Info("snapshot interval adjusted", "previous", snapshot.CreatedAt.Format(time.RFC3339), "next", nextSnapshot.String())
+	if r.SnapshotOffset > 0 {
+		logger.Info("snapshot interval adjusted", "next", time.Now().Add(r.SnapshotOffset).Format(time.RFC3339))
 
-			select {
-			case <-ctx.Done():
-				return
-			case <-time.After(nextSnapshot):
-				if _, err := r.Snapshot(ctx); err != nil && err != ErrNoGeneration {
-					logger.Error("snapshotter error", "error", err)
-				}
+		select {
+		case <-ctx.Done():
+			return
+		case <-time.After(r.SnapshotOffset):
+			if _, err := r.Snapshot(ctx); err != nil && err != ErrNoGeneration {
+				logger.Error("snapshotter error", "error", err)
 			}
 		}
 	}