Bug#35591 FLUSH PRIVILEGES caused a crash

A race condition on the privilege hash tables (proc_priv_hash
and func_priv_hash) caused one thread to try to delete elements
that had already been deleted by another thread.

The bug was caused by reading and saving the pointers to 
the hash tables outside mutex protection.  This led to an
inconsistency where a thread copied a pointer to a hash,
another thread did the same, the first thread then deleted
the hash, and the second then crashed when it in turn tried to
delete the deleted hash.

The fix is to ensure that operations on the shared hash structures
happens under mutex protection (moving the locking up a little)

The problem is not actually related to 2byte collation IDs.
The same crash happens if you change the collation ID in
mysql-test/str_data/Index.xml to a value smaller than 256.

Crash happened in SQL parser, because the "ident_map" and "state_map"
arrays were not initialized in loadable utf8 collations.

Fix: adding proper initialization of the "ident_map" and "state_map"
members for loadable utf8 collations.

Part II, engines instrumentation

Added back n_frees, use 'clear' instead of 'free' since memory is
not freed here.

Before this patch, semisync assumed transactions running in parallel
can not be larger than max_connections, but this is not true when
the event scheduler is executing events, and cause semisync run out
of preallocated transaction nodes.

Fix the problem by allocating transaction nodes dynamically.

This patch also fixed a possible deadlock when running UNINSTALL
PLUGIN rpl_semi_sync_master and updating in parallel. Fixed by
releasing the internal Delegate lock before unlock the plugins.

Add $basedir/lib/plugin to the search paths for semisync plugins.

Conflicts:
  - sql/sql_yacc.yy

Use safe output formats for strings that are not null terminated.

      
Constant expressions in WHERE/HAVING/ON clauses aren't cached and evaluated
for each row. This causes slowdown of query execution especially if constant
UDF/SP function are used.
      
Now WHERE/HAVING/ON expressions are analyzed in the top-bottom direction with
help of the compile function. When analyzer meets a constant item it
sets a flag for the tree transformer to cache the item and doesn't allow tree
walker to go deeper. Thus, the topmost item of a constant expression if
cached. This is done after all other optimizations were applied to
WHERE/HAVING/ON expressions
      
A helper function called cache_const_exprs is added to the JOIN class.
It calls compile method with caching analyzer and transformer on WHERE,
HAVING, ON expressions if they're present.
The cache_const_expr_analyzer and cache_const_expr_transformer functions are
added to the Item class. The first one check if the item can be cached and
the second caches it if so.
A new Item_cache_datetime class is derived from the Item_cache class.
It caches both int and string values of the underlying item independently to
avoid DATETIME aware int-to-string conversion. Thus it completely relies on
the ability of the underlying item to correctly convert DATETIME value from
int to string and vice versa.

Text conflict in mysql-test/collections/default.experimental
Text conflict in mysql-test/r/show_check.result
Text conflict in mysql-test/r/sp-code.result
Text conflict in mysql-test/suite/binlog/r/binlog_tmp_table.result
Text conflict in mysql-test/suite/rpl/t/disabled.def
Text conflict in mysql-test/t/show_check.test
Text conflict in mysys/my_delete.c
Text conflict in sql/item.h
Text conflict in sql/item_cmpfunc.h
Text conflict in sql/log.cc
Text conflict in sql/mysqld.cc
Text conflict in sql/repl_failsafe.cc
Text conflict in sql/slave.cc
Text conflict in sql/sql_parse.cc
Text conflict in sql/sql_table.cc
Text conflict in sql/sql_yacc.yy
Text conflict in storage/myisam/ha_myisam.cc

Corrected results for
 stm_auto_increment_bug33029.reject      2009-12-01
		20:01:49.000000000 +0300
       <andrei> @@ -42,9 +42,6 @@
       <andrei>  RETURN i;
       <andrei>  END//
       <andrei>  CALL p1();
       <andrei> -Warnings:
       <andrei> -Note   1592    Statement may not be safe to log in statement
		format.
       <andrei> -Note   1592    Statement may not be safe to log in statement
		format.

There should be indeed no Note present because there is in fact autoincrement 
top-level query in sp() that triggers inserting in yet another auto-inc table.
(todo: alert DaoGang to improve the test).

mysqld crashed in network_init()).

The problem was that current_thd was not ready at that point in mysqld life,
so ER() macro could not be used.

The fix is to use ER_DEFAULT() macro, which is intented for such cases.

Part 1: Instrumentation interface

merging from 5.1 to rep+2 starting at gca(5.1, next-mr) == build@mysql.com-20091104182209-iui387z35159aoyw