Group replica wal segments by index

This commit changes the replica path format to group segments within
a single index in the same directory. This is to eventually add the
ability to seek to a record on file-based systems without having
to iterate over the records. The DB shadow WAL will also be changed
to this same format to support live replicas.

internal/internal_test.go

@@ -0,0 +1,61 @@
+package internal_test
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/benbjohnson/litestream/internal"
+)
+
+func TestParseSnapshotPath(t *testing.T) {
+	for _, tt := range []struct {
+		s     string
+		index int
+		err   error
+	}{
+		{"00bc614e.snapshot.lz4", 12345678, nil},
+		{"xxxxxxxx.snapshot.lz4", 0, fmt.Errorf("invalid snapshot path")},
+		{"00bc614.snapshot.lz4", 0, fmt.Errorf("invalid snapshot path")},
+		{"00bc614e.snapshot.lz", 0, fmt.Errorf("invalid snapshot path")},
+		{"00bc614e.snapshot", 0, fmt.Errorf("invalid snapshot path")},
+		{"00bc614e", 0, fmt.Errorf("invalid snapshot path")},
+		{"", 0, fmt.Errorf("invalid snapshot path")},
+	} {
+		t.Run("", func(t *testing.T) {
+			index, err := internal.ParseSnapshotPath(tt.s)
+			if got, want := index, tt.index; got != want {
+				t.Errorf("index=%#v, want %#v", got, want)
+			} else if got, want := err, tt.err; !reflect.DeepEqual(got, want) {
+				t.Errorf("err=%#v, want %#v", got, want)
+			}
+		})
+	}
+}
+
+func TestParseWALSegmentPath(t *testing.T) {
+	for _, tt := range []struct {
+		s      string
+		index  int
+		offset int64
+		err    error
+	}{
+		{"00bc614e/000003e8.wal.lz4", 12345678, 1000, nil},
+		{"00000000/00000000.wal", 0, 0, fmt.Errorf("invalid wal segment path")},
+		{"00000000/00000000", 0, 0, fmt.Errorf("invalid wal segment path")},
+		{"00000000/", 0, 0, fmt.Errorf("invalid wal segment path")},
+		{"00000000", 0, 0, fmt.Errorf("invalid wal segment path")},
+		{"", 0, 0, fmt.Errorf("invalid wal segment path")},
+	} {
+		t.Run("", func(t *testing.T) {
+			index, offset, err := internal.ParseWALSegmentPath(tt.s)
+			if got, want := index, tt.index; got != want {
+				t.Errorf("index=%#v, want %#v", got, want)
+			} else if got, want := offset, tt.offset; got != want {
+				t.Errorf("offset=%#v, want %#v", got, want)
+			} else if got, want := err, tt.err; !reflect.DeepEqual(got, want) {
+				t.Errorf("err=%#v, want %#v", got, want)
+			}
+		})
+	}
+}