Refactor Restore()

This commit refactors out the complexity of downloading ordered WAL
files in parallel to a type called `WALDownloader`. This makes it
easier to test the restore separately from the download.

mock/read_closer.go

@@ -0,0 +1,14 @@
+package mock
+
+type ReadCloser struct {
+	CloseFunc func() error
+	ReadFunc  func([]byte) (int, error)
+}
+
+func (r *ReadCloser) Close() error {
+	return r.CloseFunc()
+}
+
+func (r *ReadCloser) Read(b []byte) (int, error) {
+	return r.ReadFunc(b)
+}

mock/snapshot_iterator.go

@@ -0,0 +1,28 @@
+package mock
+
+import (
+	"github.com/benbjohnson/litestream"
+)
+
+type SnapshotIterator struct {
+	CloseFunc    func() error
+	NextFunc     func() bool
+	ErrFunc      func() error
+	SnapshotFunc func() litestream.SnapshotInfo
+}
+
+func (itr *SnapshotIterator) Close() error {
+	return itr.CloseFunc()
+}
+
+func (itr *SnapshotIterator) Next() bool {
+	return itr.NextFunc()
+}
+
+func (itr *SnapshotIterator) Err() error {
+	return itr.ErrFunc()
+}
+
+func (itr *SnapshotIterator) Snapshot() litestream.SnapshotInfo {
+	return itr.SnapshotFunc()
+}

mock/wal_segment_iterator.go

@@ -0,0 +1,28 @@
+package mock
+
+import (
+	"github.com/benbjohnson/litestream"
+)
+
+type WALSegmentIterator struct {
+	CloseFunc      func() error
+	NextFunc       func() bool
+	ErrFunc        func() error
+	WALSegmentFunc func() litestream.WALSegmentInfo
+}
+
+func (itr *WALSegmentIterator) Close() error {
+	return itr.CloseFunc()
+}
+
+func (itr *WALSegmentIterator) Next() bool {
+	return itr.NextFunc()
+}
+
+func (itr *WALSegmentIterator) Err() error {
+	return itr.ErrFunc()
+}
+
+func (itr *WALSegmentIterator) WALSegment() litestream.WALSegmentInfo {
+	return itr.WALSegmentFunc()
+}