Fix for a BUG#31649: events.test fails: NULL "state" field of SHOW PROCESSLIST.
Fix for a BUG#31649: events.test fails: NULL "state" field of
SHOW PROCESSLIST.

The problem was a race condition: if the Event Scheduler was not
quick enough, the following scenario happens:
  - The Event Scheduler picks up the created event;
  - The event is executed;
  - event_scheduler_thread->proc_info is set to NULL;
  - The client issues SELECT FROM I_S.

The fix is to wait for the Event Scheduler to reach 'Waiting
for next activation' state. 

ollie314/server,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,natsys/mariadb_10.2,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,slanterns/server,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,ollie314/server,natsys/mariadb_10.2,ollie314/server,ollie314/server,davidl-zend/zenddbi,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix