refactor(ngModelController,formController): centralize and simplify logic

The previous logic for async validation in 
`ngModelController` and `formController` was not maintainable:
- control logic is in multiple parts, e.g. `ctrl.$setValidity`
  waits for end of promises and continuous the control flow
  for async validation
- logic for updating the flags `ctrl.$error`, `ctrl.$pending`, `ctrl.$valid`
  is super complicated, especially in `formController`

This refactoring makes the following changes:
- simplify async validation: centralize control logic
  into one method in `ngModelController`:
  * remove counters `invalidCount` and `pendingCount`
  * use a flag `currentValidationRunId` to separate
    async validator runs from each other
  * use `$q.all` to determine when all async validators are done
- centralize way how `ctrl.$modelValue` and `ctrl.$invalidModelValue` 
  is updated
- simplify `ngModelController/formCtrl.$setValidity` and merge
  `$$setPending/$$clearControlValidity/$$clearValidity/$$clearPending`
  into one method, that is used by `ngModelController` AND
  `formController`
  * remove diff calculation, always calculate the correct state anew,
    only cache the css classes that have been set to not
    trigger too many css animations.
  * remove fields from `ctrl.$error` that are valid and add `ctrl.$success`:
    allows to correctly separate states for valid, invalid, skipped and pending,
    especially transitively across parent forms.
- fix bug in `ngModelController`:
  * only read out `input.validity.badInput`, but not
    `input.validity.typeMismatch`,
    to determine parser error: We still want our `email` 
    validator to run event when the model is validated.
- fix bugs in tests that were found as the logic is now consistent between
  `ngModelController` and `formController`

BREAKING CHANGE:
- `ctrl.$error` does no more contain entries for validators that were
  successful. Instead, they are now saved in `ctrl.$success`.
- `ctrl.$setValidity` now differentiates between `true`, `false`,
  `undefined` and `null`, instead of previously only truthy vs falsy.