Copy authz for goroutine in RA. (#3508)

There's a minor data race in UpdateAuthorization where the authorization can
be changed in the goroutine concurrently with returning it. This fixes it by
copying the authorization as an argument to the goroutine.

Fixes #3506.

ra/ra.go

@@ -1486,7 +1486,7 @@ func (ra *RegistrationAuthorityImpl) UpdateAuthorization(ctx context.Context, ba
 	// Dispatch to the VA for service
 
 	vaCtx := context.Background()
-	go func() {
+	go func(authz core.Authorization) {
 		records, err := ra.VA.PerformValidation(vaCtx, authz.Identifier.Value, authz.Challenges[challengeIndex], authz)
 		var prob *probs.ProblemDetails
 		if p, ok := err.(*probs.ProblemDetails); ok {
@@ -1518,7 +1518,7 @@ func (ra *RegistrationAuthorityImpl) UpdateAuthorization(ctx context.Context, ba
 				"Could not record updated validation: err=[%s] regID=[%d] authzID=[%s]",
 				err, authz.RegistrationID, authz.ID))
 		}
-	}()
+	}(authz)
 	ra.stats.Inc("UpdatedPendingAuthorizations", 1)
 	return authz, nil
 }