and ON COMPLETION NOT PRESERVE is deleted at server restart.

From user's point of view when user creates event with default on
completion behaviour and its status is DISABLE,
then after restart of server this event is deleted from schema.

The problem was that during loading of events from mysql.event table
server was used incorrect implementation for 'on completion' behaviour, i.e.
if event loaded from table had status value DISABLE and ON COMPLETION flag
was equal to true, then server deleted event from table without regarding 
status value. The root of cause was at the method Events::load_events_from_db()
(this method called by Events::init() during server startup initialization).
This method reads every event from table and creates event object from 
loaded data. When event that was disabled and had the default ON COMPLETION
NOT PRESERVE behaviour was loaded from mysql.event table into
object of class Event_queue_element then it had attribute
on_completion=Event_parse_data::ON_COMPLETION_DROP and
status = Event_parse_data::DISABLED. After loading completed it tries to
put event into queue but this failed because status == DISABLE.
Then this event dropped since the condition 
'else if (drop_on_completion)' is true.

The solution is to change wrong condition. The right condition is
to check for value of attribute Event_queue_element::dropped that is set
to true when event really should dropped because it expired
(this check is implemented at
Event_queue_element::compute_next_execution_time).

This patch also fix a bug related to deleting of expired events from 
mysql.event table after server restart.

The problem was that if there was an event that expired at time T
and before that time came server was down and started again after 
time T was elapsed then expired event will never be removed.

The reason for this bug is that when the server initializes scheduler
structures at Events::init() it's never calls set_time() 
for new THD object and so THD::start_time attribute has value 0.
Afterwards when events is loaded from mysql.event table into memory
server checks end_time of event against value of 
current_thd->query_start() that returns value 0. As a result expired
events is considered as expired.

The solution is to call THD::set_time() for new created THD at Events::init.