changefeedccl: job-level retry when error message is about draining

See #https://github.com/cockroachlabs/support/issues/1839. The flow
retryable error marker doesn't survive every path by which it can
bubble up, so just look for the single word "draining" as false positives
are much better than false negatives.

Release note (enterprise change): Fixed a bug that could cause changefeeds to fail during a rolling restart.

pkg/ccl/changefeedccl/changefeedbase/errors.go

@@ -124,7 +124,9 @@ func IsRetryableError(err error) bool {
 	// that we can't recover the structure and we have to rely on this
 	// unfortunate string comparison.
 	errStr := err.Error()
-	if strings.Contains(errStr, retryableErrorString) || strings.Contains(errStr, kvcoord.SendErrorString) {
+	if strings.Contains(errStr, retryableErrorString) ||
+		strings.Contains(errStr, kvcoord.SendErrorString) ||
+		strings.Contains(errStr, "draining") {
 		return true
 	}