Add ignore() method to to InsertQuery_mysql class

MySQL's insert query supports the IGNORE keyword, which is describe in the MySQL manual:

If you use the IGNORE keyword, errors that occur while executing the INSERT statement are ignored. For example, without IGNORE, a row that duplicates an existing UNIQUE index or PRIMARY KEY value in the table causes a duplicate-key error and the statement is aborted. With IGNORE, the row still is not inserted, but no error occurs. Ignored errors may generate warnings instead, although duplicate-key errors do not.

Currently, one cannot use db_insert() with this keyword—no method exists for attaching it to the query string. I'd like to consider adding an ignore() method to InsertQuery_mysql which will append ' IGNORE' to the end of insert queries.

This has at least one drawback: it is only applicable to the mysql db engine. We'd only be adding it to the InsertQuery_mysql class, but the mere existence of this method for one db engine may cause module creators to inadvertently make modules dependent on the MySQL backend.

A number of links to places where people have requested this functionality:

• http://drupal.stackexchange.com/questions/11177/how-do-i-use-ignore-in-a...
• http://drupal.stackexchange.com/questions/89253/how-to-set-insert-ignore...
• https://drupal.org/node/887170
• https://drupal.org/node/1986706