Support upgrade --dry-run

[Ehco1996]

Feature Request

Is your feature request related to a problem? Please describe:

Describe the feature you'd like:

Before upgrading the dm cluster, use the upgrade -dry-run command to check if the upgrade will be successful

1. Make sure all tasks are suspended before upgrading

2. Connect to the etcd of the current cluster and copy all the keys to a new prefix

3. Start a new version dm-master and check the status of all keys with the keys copied in the previous step

4. clear the second copied key if no abnormalities are found and stop the new server

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: