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.


mysql-test/r/trigger-trans.result:
  Update results (Bug#26141)
mysql-test/r/trigger.result:
  Update results (Bug#28502)
mysql-test/t/trigger-trans.test:
  Add a test case for Bug#26141 mixing table types in trigger causes 
  full table lock on innodb table.
mysql-test/t/trigger.test:
  Add a test case for Bug#28502 Triggers that update another innodb 
  table will block echo on X lock unnecessarily. Add more test 
  coverage for triggers.
sql/item.h:
  enum trg_event_type is needed in table.h
sql/sp.cc:
  Take into account table_list->trg_event_map when determining
  what tables to pre-lock. 
  
  After this change, if we attempt to fire a 
  trigger for which we had not pre-locked any tables, error
  'Table was not locked with LOCK TABLES' will be printed.
  This, however, should never happen, provided the pre-locking
  algorithm has no programming bugs.
  
  Previously a trigger key in the sroutines hash was based on the name 
  of the table the trigger belongs to. This was possible because we would
  always add to the pre-locking list all the triggers defined for a table when
  handling this table.
  Now the key is based on the name of the trigger, owing
  to the fact that a trigger name must be unique in the database it
  belongs to.
sql/sp_head.cc:
  Generate sroutines hash key in init_spname(). This is a convenient
  place since there we have all the necessary information and can
  avoid an extra alloc.
  
  Maintain and merge trg_event_map when adding and merging elements
  of the pre-locking list.
sql/sp_head.h:
  Add ,m_sroutines_key member, used when inserting the sphead for a
  trigger into the cache of routines used by a statement.
  Previously the key was based on the table name the trigger belonged
  to, since for a given table we would add to the sroutines list
  all the triggers defined on it.
sql/sql_lex.cc:
  Introduce a new lex step: set_trg_event_type_for_tables().
  It is called when we have finished parsing but before opening
  and locking tables. Now this step is used to evaluate for each
  TABLE_LIST instance which INSERT/UPDATE/DELETE operation, if any,
  it is used in.
  In future this method could be extended to aggregate other information
  that is hard to aggregate during parsing.
sql/sql_lex.h:
  Add declaration for set_trg_event_type_for_tables().
sql/sql_parse.cc:
  Call set_trg_event_type_for_tables() after MYSQLparse(). Remove tabs.
sql/sql_prepare.cc:
  Call set_trg_event_type_for_tables() after  MYSQLparse().
sql/sql_trigger.cc:
  Call set_trg_event_type_for_tables() after MYSQLparse().
sql/sql_trigger.h:
  Remove an obsolete member.
sql/sql_view.cc:
  Call set_trg_event_type_for_tables() after MYSQLparse().
sql/sql_yacc.yy:
  Move assignment of sp_head::m_type before calling sp_head::init_spname(), 
  one is now used inside another.
sql/table.cc:
  Implement TABLE_LIST::set_trg_event_map() - a method that calculates
  wh triggers may be fired on this table when executing a statement.
sql/table.h:
  Add missing declarations.
  Move declaration of trg_event_type from item.h (it will be needed for 
  trg_event_map bitmap when we start using Bitmap template instead
  of uint8).