Removed - persistent from deps

The standalone peristent is incompatible with the one provided by Zope
Removed - redundant events

These were feature creeping. First release is to be bare minimum for our
needs
Changes to the utility

Removed - remaining uses and time_to_live as they're beyond scope
Added - valid method
Changed - parameter names for generate_new_token to be more explicit
Changed - expiry to default to datetime.max for infinte life tokens
Added - _consume_token implementation
Renamed parameter to be more explicit in its meaning
Fixed test
Removed - redundant tests

Added - test_valid
Expanded _consume_token test

Not only testing the method, also testing the event is fired
Added - tests for private methods