move back THD::change_security_context to event_change_security_context

Final stroke, events should be loaded from disk on server startup.
Also check the validity of their bodies if possible during loading.

event_scheduler_ng.cc/h is no more

More small fixes to the API : use LEX_STRING instead of LEX_STRING* and if error
then return bool(true) instead of error code.
Merged functions. Reduced usage of sp_name.
Fixed a lot of function documentation errors.
Added function documentation wherever needed.
Removed some unused defines and error codes.

Next to come is batch rename of Event_scheduler_ng to Event_scheduler.

After merge fixes.

This patch introduces specialized Event data objects
Event_basic as parent.
Event_queue_element used for queue storage
Event_timed used for SHOW EVENTS/ I_S.EVENTS / SHOW CREATE EVENT
Event_job_data using during execution.
Methods were moved out of Event_timed to other classes.

This patch also introduces Events::LOCK_event_metadata.
This patch gives new implementation of Events::dump_internal_status().
Now both the Event_scheduler and Event_queue return information during
their ::dump_internal_status().

Shortened a bit the runtime for executing events test cases.

into  jabberwock.site:/home/dlenev/mysql-5.1-bg18437

(the bug itself is not repeatable any more).

into  jabberwock.site:/home/dlenev/mysql-5.1-bg18437

into  bodhi.local:/opt/local/work/mysql-5.1-runtime-merge-with-5.0

into  poseidon.ndb.mysql.com:/home/tomas/mysql-5.1-ndb

- keep accounting for aoutstanding drop trigger requests
- also lock table object in suma while doing that (should be impossible right now though since dict serializes all requests)

into  jabberwock.site:/home/dlenev/mysql-5.1-bg18437

into  bodhi.netgear:/opt/local/work/mysql-5.1-runtime-merge-with-5.0

into  bodhi.netgear:/opt/local/work/mysql-5.1-runtime-merge

removed alter table partition error test since it cannot be made predictable... name will change of created file

into  poseidon.ndb.mysql.com:/home/tomas/mysql-5.1-ndb

into  bodhi.netgear:/opt/local/work/mysql-5.1-runtime-merge-with-5.0

into  bodhi.netgear:/opt/local/work/mysql-5.1-runtime-merge

into  poseidon.ndb.mysql.com:/home/tomas/mysql-5.1-ndb

into trift2.:/M51/bug20216-5.1

into trift2.:/M51/merge-5.1

into  poseidon.ndb.mysql.com:/home/tomas/mysql-5.1-ndb

- missing order by

into  poseidon.ndb.mysql.com:/home/tomas/mysql-5.1-ndb

into  poseidon.ndb.mysql.com:/home/tomas/mysql-5.0

- don't invalidate tables that are in state RETRIEVING

into  perch.ndb.mysql.com:/home/jonas/src/mysql-5.1-new-ndb

into  poseidon.ndb.mysql.com:/home/tomas/mysql-5.1-ndb

before update trigger on NDB table".

Two main changes:
- We use TABLE::read_set/write_set bitmaps for marking fields used by
  statement instead of Field::query_id in 5.1.
- Now when we mark columns used by statement we take into account columns 
  used by table's triggers instead of marking all columns as used if table
  has triggers.

into  poseidon.ndb.mysql.com:/home/tomas/mysql-5.1-ndb

into  perch.ndb.mysql.com:/home/jonas/src/mysql-5.1-new-ndb