This decoupling allows in further versions of MySQL enum interval_type to
  be reordered without this affecting any backward compatibility in the
  events code.
  This changeset doesn't change any exposed behavior but makes events' code
  more durable to changes outside of their code base.
  
  To the reviewer: There is no regression test included as it is impossible
  to construct one with the current infrastructure which can test it. To test
  the code one has create and event, then change the order of
  enum interval_type in my_time.h, update sql/time.cc, recompile the server
  and run it with scheduler running.


include/my_time.h:
  Add a reminder to keep enum interval_type and
  interval_type_to_name in sync
sql/event_data_objects.cc:
  When loading from disk don't use the integer value of
  mysql.event.interval_field because it could be different of the
  values of enum interval_type, if the latter is reordered in a later
  version of MySQL. Loaded from disk is the string value which is then
  resolved against interval_type_to_name to get the exact enum value we
  need for Event_queue_element::interval.
sql/event_db_repository.cc:
  Use interval_type_to_name from sql/time.cc during storage thus
  decoupling the value stored on disk (the enum) from the integer
  representation in memory. interval_type can be changed and as
  long as interval_type_to_name is kept in sync correct values will
  be saved to disk.
sql/mysql_priv.h:
  add proto of find_string_in_array
sql/strfunc.cc:
  Add a function for searching a LEX_STRING in an array of LEX_STRINGs