tube:grant() does not grant privileges to all public api methods

[rybakit]

So far, I've found the following methods that lack privileges:

• tube:truncate()
• queue.stats()
• queue.statistics()

[Totktonada]

Cite from README:

• tube:truncate() - delete all tasks from the tube. Note that tube:truncate
  must be called only by the user who created this tube (has space ownership) OR
  under a setuid function. Read more about setuid functions
  here.

[Totktonada]

Raw idea: We can provide an option to control whether appropriate priveleges should be granted for :truncate().