causes full table lock on innodb table.
Also fixes Bug#28502 Triggers that update another innodb table 
will block on X lock unnecessarily (duplciate).
Code review fixes.

Both bugs' synopses are misleading: InnoDB table is
not X locked. The statements, however, cannot proceed concurrently, 
but this happens due to lock conflicts for tables used in triggers,
not for the InnoDB table. 

If a user had an InnoDB table, and two triggers, AFTER UPDATE and 
AFTER INSERT, competing for different resources (e.g. two distinct
MyISAM tables), then these two triggers would not be able to execute
concurrently. Moreover, INSERTS/UPDATES of the InnoDB table would
not be able to run concurrently. 
The problem had other side-effects (see respective bug reports).

This behavior was a consequence of a shortcoming of the pre-locking
algorithm, which would not distinguish between different DML operations
(e.g. INSERT and DELETE) and pre-lock all the tables
that are used by any trigger defined on the subject table.

The idea of the fix is to extend the pre-locking algorithm to keep track,
for each table, what DML operation it is used for and not
load triggers that are known to never be fired.