Add maintenance window logic

[ericaporter]

Relating to this ticket

We need the ability to do maintenance on on a live streamed BiqQuery dataset, such as register_events_production.

This PR adds a configuration item for a maintenance window.

Redis logic - WIP

• On reaching the start maintenance window start timestamp, DfE Analytics will start queueing items with a schedule time of:
  end_timestamp + number of seconds elapsed since start_timestamp
  The queued items will end up queued in Redis.
• Once the end of the maintenance window is reached, the queue items will be processed and sent to BigQuery and a First In First Out (FIFO) base.

[asatwal]

See comments in code on suggestions for implementation.

[asatwal]

Need to remove this block here and move into the do method above.

Suggested change

	if DfE::Analytics.within_maintenance_window?
	# Delaying / Queueing events - the redis bit
	Rails.logger.info('Within BigQuery maintenance window. Events will be queued for later processing.')
	return
	end

In the do method replace lines 13-17 with the following:

   if DfE::Analytics.within_maintenance_window?
     set(wait_until: DfE::Analytics.next_scheduled_time_after_maintenance_window).perform_later(events)
   elsif DfE::Analytics.async?
     perform_later(events)
   else
     perform_now(events)
   end

[asatwal]

Looks good @ericaporter - Just have a minor comment about adding another test.