Triggers are implemented as collection of items per dd::Table
object.

A) Following are the signature of new DD API's implemented:

  * Check if table has triggers.

    virtual bool has_trigger() const = 0;

  * Get const reference to Trigger_collection.

    virtual const Trigger_collection&
    dd::Table::triggers() const = 0;

  * Get non-const pointer to Trigger_collection.

    virtual Trigger_collection *dd::Table::triggers() = 0;

  * Add new trigger.

    virtual Trigger *
    add_trigger(Trigger::enum_action_timing at,
                Trigger::enum_event_type et) = 0;

  * Get Trigger object for the given trigger name.

    virtual const Trigger*
    get_trigger(const char *name) const = 0;

  * Add new trigger just after the trigger specified in argument.

    virtual Trigger *
    add_trigger_following(const Trigger *trigger,
                          Trigger::enum_action_timing at,
                          Trigger::enum_event_type et) = 0;

  * Add new trigger just before the trigger specified in argument.

    virtual Trigger *
    add_trigger_preceding(const Trigger *trigger,
                          Trigger::enum_action_timing at,
                          Trigger::enum_event_type et) = 0;

  * Drop the given trigger object.

    virtual void drop_trigger(Trigger *trigger) = 0;

  * Drop all triggers.

    virtual void drop_all_triggers() = 0;

  * Get table name of a given trigger name.

    dd::cache::Dictionary_client::
      get_table_name_by_trigger_name(
        Object_id schema_id,
        const std::string &trigger_name,
        std::string *table_name);

B) Following section highlight the steps to carry on various
operations on mysql.triggers DD table.

  i) Steps to add trigger:
  - Get dd::Table object for which you want to add trigger.
  - Invoke dd::Table::add_trigger*() API.
  - Set dd::Trigger attributes.
  - Store the dd::Table object. All the trigger objects gets
    stored in mysql.triggers.

  ii) Steps to get dd::Table for a given trigger name:
  - Invoke
    Dictionary_client::get_table_name_by_trigger_name()
    with a trigger name to get the table name for which
    the trigger belongs to.
  - Make sure you acquire desired MDL lock on table name.
  - Get dd::Table object using Dictionary_client::acquire<>().

  iii) Steps to drop trigger:
  - Follow steps to get dd::Table given trigger name above.
  - Use dd::Table::get_trigger(<trigger_name>) API to get the
    dd::Trigger object.
  - Invoke dd::Table::drop_trigger(Trigger*) to remove the
    trigger on the Table.
  - Store dd::Table object, which will remove the trigger from
    mysql.triggers DD table.

C) Implemented a method to move all triggers from a table to another.
   This is mostly used during ALTER TABLE.

  Triggers from from_schema_name.from_table_name will be moved into
  to_schema_name.to_table_name and the transaction will be committed.

    bool dd::move_triggers(THD *thd,
                           const char *from_schema_name,
                           const char *from_name,
                           const char *to_schema_name,
                           const char *to_name);