domaininfo: Reload periodically

This patch makes chasquid reload domaininfo periodically, so it notices
any external changes made to it.

It is in line with what we do for aliases and authentication already,
and makes it possible for external removals an additions to the
domaininfo database to be picked up without a restart.

internal/testlib/testlib.go

@@ -37,3 +37,17 @@ func RemoveIfOk(t *testing.T, dir string) {
 		os.RemoveAll(dir)
 	}
 }
+
+func Rewrite(t *testing.T, path, contents string) error {
+	// Safeguard, to make sure we only mess with test files.
+	if !strings.Contains(path, "testlib_") {
+		panic("invalid/dangerous path")
+	}
+
+	err := ioutil.WriteFile(path, []byte(contents), 0600)
+	if err != nil {
+		t.Errorf("failed to rewrite file: %v", err)
+	}
+
+	return err
+}