More trigger ddl support:
+ add support for show triggers
+ add support for show create trigger
+ add support for information_schema.triggers

Store the original raw sql alongside the body so that we can handle show create trigger.
Add a list of triggers into UserTable, make sure we maintain the collection appropriately.
Add PECreateTriggerStatement, make it persist PETable as the root and invalidate PETable.
Add a bunch of necessary fields to triggers (i.e. connection charset/collation, sql_mode, etc)
Make any stmt fail that would cause a trigger to fire.