Reduce snapshot check frequency

Previously, a bug was introduced that added a `LIST` operation
on every replica sync which significantly increased the cost of
running Litestream against S3. This changes the behavior to only
issue the `LIST` operation when the generation has changed.

litestream.go

@@ -207,13 +207,11 @@ func FilterSnapshotsAfter(a []SnapshotInfo, t time.Time) []SnapshotInfo {
 // FindMinSnapshotByGeneration finds the snapshot with the lowest index in a generation.
 func FindMinSnapshotByGeneration(a []SnapshotInfo, generation string) *SnapshotInfo {
 	var min *SnapshotInfo
-	for i := range a {
-		snapshot := &a[i]
-
+	for _, snapshot := range a {
 		if snapshot.Generation != generation {
 			continue
 		} else if min == nil || snapshot.Index < min.Index {
-			min = snapshot
+			min = &snapshot
 		}
 	}
 	return min

litestream_test.go

@@ -128,16 +128,6 @@ func TestWALSegmentPath(t *testing.T) {
 	})
 }
 
-func TestFindMinSnapshotByGeneration(t *testing.T) {
-	infos := []litestream.SnapshotInfo{
-		{Generation: "29cf4bced74e92ab", Index: 0},
-		{Generation: "5dfeb4aa03232553", Index: 24},
-	}
-	if got, want := litestream.FindMinSnapshotByGeneration(infos, "29cf4bced74e92ab"), &infos[0]; got != want {
-		t.Fatalf("info=%#v, want %#v", got, want)
-	}
-}
-
 func MustDecodeHexString(s string) []byte {
 	b, err := hex.DecodeString(s)
 	if err != nil {