Cron uses the wrong timestamp method

[AntonEvers]

Description

Because the cron job uses a localized timestamp instead of a regular
timestamp, all times in the cron_schedule table are not the actual
times, but times with a double offset.

By using the regular timestamp, all times in cron_schedule are correct.
The cron job execution times are not affected by this change because
schedule generation and job execution depend on the same timestamp.
If both are in the same timezone all will go as intended.

Fixes #4237

Fixed Issues (if relevant)

1. Cron times in the database have a double timezone correction #4237: Cron times in the database are not UTC

Manual testing scenarios

1. truncate the cron_schedule table
2. flush the caches
3. execute bin/magento cron:run
4. verify that the earliest created_at time in the database is the current time
5. wait for one minute
6. execute bin/magento cron:run again
7. verify that the earliest executed_at time in the database is the current time
8. verify that only the jobs that are supposed to run based on their scheduled_at time, have run

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)