Actually it is not a bug but right behavior observed as pefomance 
degradation after we have forced Item_field::fix_fields() to 
re-execute each time when we are executing prep stmt.

This patch implements small optimization which heals this bad 
behavior. We are caching field position in TABLE::field array in
Item's member and are using this position for speeding up field
lookups in fix_fields() in case of its re-execution.


sql/item.cc:
  Added cached_field_index member to Item_ident for caching field
  position in TABLE::field array for quicker field lookup in case 
  fix_fields() is executed twice.
sql/item.h:
  Added cached_field_index member to Item_ident for caching field
  position in TABLE::field array for quicker field lookup in case 
  fix_fields() is executed twice.
sql/mysql_priv.h:
  Now find_field_in_table has one more in-out parameter which is used
  for caching field index in TABLE::field array stored for quicker
  field look up.
sql/sql_acl.cc:
  Now find_field_in_table has one more in-out parameter which is used
  for caching field index in TABLE::field array stored for quicker
  field look up.
sql/sql_base.cc:
  Optimization: Now when we are looking up Field by its name we are 
  caching its position in TABLE::field array in Item_ident::cached_field_index,
  so when we will call fix_fields() second time for this item (e.g.
  when we are re-executing prep statement) we can reuse this index
  to speed up this lookup.
sql/table.cc:
  Now we storing in TABLE::name_hash pointers to elements of 
  TABLE::field array, because this allows easily retrieve index
  of field in this array when it is looked up by its name.
sql/table.h:
  Added comment claryfying what we store in TABLE::name_hash.
BitKeeper/etc/logging_ok:
  Logging to logging@openlogging.org accepted