WL#9644: Enable event_scheduler by default

Affects: Server-8.0   —   Status: Complete   —   Priority: Medium

The event scheduler is OFF by default, which means that not many users will be 
using it.

There are a number of features possible in the SYS schema that are not currently 
done (such as kill idle transactions) because of this.

This WL will change the default to event_scheduler=ON.
Functional Requirement:

- Event scheduler should be ON by default when the server starts.
User Visible Impact:
--------------------

"SHOW PROCESSLIST" command will show an extra entry by default for event scheduler.

     Id: <>
   User: event_scheduler
   Host: localhost
     db: NULL
Command: Daemon
   Time: 43
  State: Waiting on empty queue
   Info: NULL


Documentation Links:
--------------------
-
https://dev.mysql.com/doc/refman/5.7/en/server-options.html#option_mysqld_event-scheduler
-
https://dev.mysql.com/doc/refman/5.7/en/events-configuration.html#events-event-scheduler-option