Basic rate limiting implementation.

Added `--limit-upload` and `--limit-download` flags to rate limit
backups and restores.

internal/limiter/limiter.go

@@ -0,0 +1,17 @@
+package limiter
+
+import (
+	"io"
+)
+
+// Limiter defines an interface that implementors can use to rate limit I/O
+// according to some policy defined and configured by the implementor.
+type Limiter interface {
+	// Upstream returns a rate limited reader that is intended to be used in
+	// uploads.
+	Upstream(r io.Reader) io.Reader
+
+	// Downstream returns a rate limited reader that is intended to be used
+	// for downloads.
+	Downstream(r io.Reader) io.Reader
+}