This also should fix 17493 and 17346, and probably 16397 (not tested).
WL#1034 (Internal CRON)
(post-review commit)


mysql-test/r/events.result:
  update result
mysql-test/t/events.test:
  update test
sql/event.cc:
  seems we get a crash if we compile it this way. better let the worker thread do it,
  this increases the stability.
sql/event_executor.cc:
  kick in more often
sql/event_timed.cc:
  - don't NULLify last_executed
  - implement (fix) better get_next_time() which does not use last_executed as basis
    but STARTS. STARTS is used as basis and the next point of time which is before ENDS,
    if set, is being found. The point > now and (point-starts) % interval_expression == 0.
sql/item_timefunc.cc:
  move calc_time_diff to time.cc as it can be reused in other parts of the 
  server code (see event_timed.cc)
sql/mysql_priv.h:
  export calc_time_diff() moved to time.cc from item_timefunc.cc
sql/sql_show.cc:
  - fix presenting of LAST_EXECUTED
  - cleanup a bit
sql/time.cc:
  - move calc_time_diff() from item_timefunc.cc to here