MDEV-6112 multiple triggers per table

This is similar to MysQL Worklog 3253, but with
a different implementation. The disk format and
SQL syntax is identical with MySQL 5.7.

Fetures supported:
- "Any" ammount of any trigger
- Supports FOLLOWS and PRECEDES to be
  able to put triggers in a certain execution order.

Implementation details:
- Class Trigger added to hold information about a trigger.
  Before this trigger information was stored in a set of lists in
  Table_triggers_list and in Table_triggers_list::bodies
- Each Trigger has a next field that poinst to the next Trigger with the
  same action and time.
- When accessing a trigger, we now always access all linked triggers
- The list are now only used to load and save trigger files.
- MySQL trigger test case (trigger_wl3253) added and we execute these
  identically.
- Even more gracefully handling of wrong trigger files than before. This
  is useful if a trigger file uses functions or syntax not provided by
  the server.
- Each trigger now has a "Created" field that shows when the trigger was
  created, with 2 decimals.

Other comments:
- Many of the changes in test files was done because of the new "Created"
  field in the trigger file. This shows up in SHOW ... TRIGGER and when
  using information_schema.trigger.
- Don't check if all memory is released if on uses --gdb;  This is needed
  to be able to get a list from safemalloc of not freed memory while
  debugging.
- Added option to trim_whitespace() to know how many prefix characters
  was skipped.
- Changed a few ulonglong sql_mode to sql_mode_t, to find some wrong usage
  of sql_mode.

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