LOG, EVENTS

Events that get dropped as part of ON COMPLETION NOT PRESERVE are
binlogged with a  XID of zero. Events that are dropped as part of
the event scheduler invocation doesn't have query_id value set.
XID generation depends on THD::query_id and hence this results in
xid value of zero.

The fix is to set the THD::query_id for the DROP EVENT sql constructed
by Event Scheduler..