consider using sub commands to use dmctl easily

[siddontang]

Feature Request

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

» help
DM control

Usage:
  dmctl [command]

Available Commands:
  break-ddl-lock       forcefully break DM-worker's DDL lock
  check-task           check the config file of the task
  help                 help about any command
  migrate-relay        migrate DM-worker's relay unit
  pause-relay          pause DM-worker's relay unit
  ......

As we can see, DM ctl has many commands, and it is hard for the user to understand them at the same time.

Describe the feature you'd like:

We can use subcommands to simplify the usage of ctl, https://github.com/spf13/cobra can help it directly, like:

task pause
task start
relay migrate
relay pause

I think this is more better.

Describe alternatives you've considered:
No

Teachability, Documentation, Adoption, Migration Strategy:

We must change the doc of DM ctl at the same time too.