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)
+}