Remove BC break
Fix polyfill support issue in CSRF validator

- typehint on AbstractContainer
- when instantiating a Container instance, use fully qualified class name in
  order to trigger alias
Typehint on AbstractContainer, but instantiate Container

- Imports will work against class_alias
- Typehinting works, *except* when using mocks, as mocks do not take
  into account class aliasing.
Revert changes to CSRF validator

- No longer necessary
Merge pull request #3869 from bakura10/remove-identical-bc

Remove BC break in Identical validator