does not work well together". Now using simplier and more correct
implementation of st_lex::unlink_first_table()/link_first_table_back() 
(It also nicely handles case when global table list is created because
of implictly used time zone tables). (2nd attempt)

Fix for bug #7705 "CONVERT_TZ() crashes with subquery/WHERE on index
column". Implemented new approach for caching objects for constant
time zone arguments. Now instead of determining whenever these arguments
are constants and performing time zone lookup at fix_fields() stage, we
do it on first get_date() invocation.

Cleanup of global @@time_zone variable handling.


mysql-test/r/timezone2.result:
  Added test for bugs #7705 "CONVERT_TZ() crashes with subquery/WHERE on
  index column" and #7899 "CREATE TABLE .. SELECT .. and CONVERT_TZ()
  function does not work well together".
mysql-test/t/timezone2.test:
  Added test for bugs #7705 "CONVERT_TZ() crashes with subquery/WHERE on
  index column" and #7899 "CREATE TABLE .. SELECT .. and CONVERT_TZ()
  function does not work well together".
sql/item_timefunc.cc:
  Item_func_convert_tz():
    New approach for caching objects for constant time zone arguments.
    Now instead of determining whenever these arguments are constants
    and performing time zone lookup at fix_fields() stage, we do it
    on first get_date() invocation. This works better in cases when 
    const_item() for these arguments returns true only on get_date()
    stage but not on fix_fields() stage (e.g. this happens in quries
    with joins or derived tables).
sql/item_timefunc.h:
  Item_func_convert_tz():
    Added from_tz_cached/to_tz_cached members indicating whenever we
    already have Time_zone object representing one of constant time zone
    arguments.
sql/set_var.cc:
  Cleaned up global @@time_zone variable handling. Now we use proper
  locking when we are setting or reading its value.
sql/set_var.h:
  Removed declaration of sys_var_thd_time_zone::get_tz_ptr() method, which
  no longer used.
sql/sql_lex.cc:
  st_lex::unlink_first_table(), st_lex::link_first_table_back():
   Simplify implementation according to Monty's suggestion.
   Instead doing something special if global and local table lists
   are the same, we simply save/restore pointers to first elements
   of both global and local lists (which works even when this lists
   are the same!). This handles nicely the case when we have separate
   global table list becuase time zone tables are implicitly used.
sql/tztime.cc:
  Backport of Monty's fixes from 5.0, which give us nicer error messages
  if we haven't found time zone with such name or its description.