Remove casts from REQUEST_TIME

Problem/Motivation

#1209470: REQUEST_TIME is a float with microseconds on PHP 5.4 introduced casts on REQUEST_TIME because of a temporary change in PHP5.4. We can remove them.

Proposed resolution

- remove the cast from the define()
- remove casts from usages of $_SERVER['REQUEST_TIME']
- remove casts from usages of $this->requestStack->getMasterRequest()->server->get('REQUEST_TIME') (and similar usages where REQUEST_TIME is retrieved from something injected or \Drupal::request() or \Drupal::requestStack()).

Remaining tasks

User interface changes

API changes

Data model changes