Implementation of Monty's idea about clear_alloc_root() optimization and cleanup of work

with memory roots in THD/Statement/Item_arena.
Added assertions preventing memory allocation on bzero'ed MEM_ROOT since it is worked by 
pure luck and was very ineffective.


include/my_sys.h:
  Reimplementation of Monty's optimization of clear_alloc_root().
  Now clear_alloc_root() can be used only for detaching memory associated with MEM_ROOT
  (e.g. to avoid its freeing). It can not be used for MEM_ROOT initialization any longer 
  (it was bad idea anyway since memory allocation on such MEM_ROOT was very ineffective 
  and worked by pure luck).
  Introduced ALLOC_ROOT_MIN_BLOCK_SIZE constant.
mysys/my_alloc.c:
  Added description of init_alloc_root().
  Added assertions to alloc_root() and reset_root_defaults() so now they can only be used
  on previosly initialized MEM_ROOT. (It worked for bzeroed MEM_ROOT before but 
  by pure luck and very inefficiently). Calling free_root() on bzero'ed MEM_ROOT
  is still ok (we can't remove this easily because of static MEM_ROOTs).
  Also now using ALLOC_ROOT_MIN_BLOCK_SIZE constant inside these functions.
sql/opt_range.cc:
  Fixed get_quick_select_for_ref() function to not use bzero'ed MEM_ROOT for allocation.
  Also QUICK_RANGEs created in this function should be created in memory root of QUICK_SELECT.
sql/sql_class.cc:
  Implementation of Monty's idea about clear_alloc_root() optimization and cleanup of work 
  with memory roots in THD/Statement/Item_arena.
  Now we are always initing THD::transaction.mem_root and THD::mem_root in THD constructor 
  (without memory allocation and with minimal block size) and then later change their
  parameters in THD::init_for_queries() (this is partially because we can't allocate anything
  on bzero'ed memory roots anymore).
  Item_arena() constructor is now trivial and is used only then Item_arena is created as backup
  storage for other Item_arena (we use Item_arena(bool) now if it is part of Statement).
  Both trivial Item_arena constructor and destructor are now inline.
  Removed unneeded clear_alloc_root from Item_arena::restore_backup_item_arena().
sql/sql_class.h:
  Both trivial Item_arena constructor and destructor are now inline.
  Commented various Item_arena constructors.

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