Make the Watchdog#restart() method private

[ma2ciek]

Suggested merge commit message (convention)

Other: Made the Watchdog#restart() method private. Changed the signatures of Watchdog#create() and Watchdog#destroy(), so now these methods will return empty promises. Closes ckeditor/ckeditor5#4701.

[Reinmar]

The PR description says that you've made restart() private so I expected to see a simple change in the docs and an underscore being added here and there. The proposed changes are much bigger though, so I'd like to understand why – was the scope of this ticket increased or I don't understand it.

[ma2ciek]

These are the only things I made within this PR:

• I moved the restart() method to the bottom and I renamed it to _restart().
• I stopped using the _restart() method in tests (as it became private), now I throw an error to perform the restart action instead.
• Since the watchdog is accessible after creation and destruction, there's no need to use watchdog.create().then( watchdog => {} ) and watchdog.destroy().then( watchdog => {} ), so I simplified these implementations and made these methods return empty promises. (This was rather a cosmetical change, I don't think anyone would use this feature).