MDL-42084 Core: Create new static class for use instead of set_time_limit
MDL-42084 Backup: Update time limit to avoid regression, use new class
MDL-42084 Core: Change set_time_limit to core_php_time_limit::raise everywhere

Exceptions (places where set_time_limit is still left) are:

Third-party libraries:
  ADODB
  PHPMailer

Moodle core:
  PHPUnit - not relevant (CLI mode)
MDL-42084 Unit tests: Remove unnecessary 'clear time limit change' lines