Replayer may panic when backend disconnects

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Restart TiDB when replaying audit logs

2. What did you expect to see? (Required)

Replayer continues replaying

3. What did you see instead (Required)

Replayer stucks because some goroutines panic

4. What is your version? (Required)

master

[djshow832]

This is because BackendConnMgr.ExecuteCmd() returns non-error when it's closing.