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/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()
+}