Maintenance Core Bug Fix & Improvements

[louislam]

Trying to fix the big issue of maintenance #2987, but realized that the schedule logic is over complicated and hard to fix, so I decided to change the core to use cron instead, which make the logic a lot easier.

• Change to cron base. No long need to generate timeslots.
• Improve isUnderMaintenance() without heavy queries.
• Allow to use cron expression directly to set a schedule
• Better Timezone handling especially for DST
• No longer need to specify the date range in case you want to schedule it endlessly.