multiple owner might exist at same time if keys are delete manually

[D3Hunter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

• 2 tidb
• tidb1 become owner, but before it watch in here, the key is deleted, so it doesn't know it should retire
  

  tidb/pkg/owner/manager.go

  Line 449 in 2088b34

  watchCh := m.etcdCli.Watch(ctx, key)

• tidb2 become owner

we should watch since the revision when we become owner to get notified

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master, and older version