WIP: refactor(ngModelController,formController):

I am assigned to following issues and found that the current
logic for async validation in ngModelController and formController
is not maintainable:
- control logic is in multiple parts, e.g. `ctrl.$setValidity`
  waits for end of promises
- logic for updating the flags `ctrl.$error`, `ctrl.$pending`, `ctrl.$valid`
  is super complicated, especially in formController


I made the following changes (almost through, only some tests are still
failing):
- simplify async validation: centralize control logic
  into one method
  * remove counters `invalidCount` and `pendingCount`
  * use a flag `currentValidationRunId` to separate
    async validator runs from each other
  * use `$q.all` to determine when all async validators are done
  * remove fields from `ctrl.$error` that are valid,
    instead of leaving them there with value `false`
    -> make logic more consistent
  * 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.
- centralize way how `ctrl.$modelValue` and `ctrl.$invalidModelValue` 
  is updated
- simplify `ngModelController/formCtrl.$setValidity` and merge
  `$$setPending`, `$$clearControlValidity/$$clearValidity/$$clearPending`
  into one method, that has the same code in `ngModelController` AND
  `formController`
- minor bug fixes encountered in the tests because of this centralization:
  * only read out `input.validity.badInput`, but not `input.validity.typeMismatch`,
    as parser error (leads to problems when using `type=email`)

@matsko What do you think?
/cc @IgorMinar 
pushing invalid values to model by default would be a huge, hard-to-track-down breaking change. I don't think that we can do it. There is a PR (possibly already merged) for ngModelOptions to make this behavior an opt-in.