Add cache

This commits adds rudimentary support for a cache directory, enabled by
default. The cache directory is created if it does not exist. The cache
is used if there's anything in it, newly created snapshot and index
files are written to the cache automatically.

internal/cache/testing.go

@@ -0,0 +1,20 @@
+package cache
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/restic"
+	"github.com/restic/restic/internal/test"
+)
+
+// TestNewCache returns a cache in a temporary directory which is removed when
+// cleanup is called.
+func TestNewCache(t testing.TB) (*Cache, func()) {
+	dir, cleanup := test.TempDir(t)
+	t.Logf("created new cache at %v", dir)
+	cache, err := New(restic.NewRandomID().String(), dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return cache, cleanup
+}