local storage for query cache). 
We need more than one pointer in a thread to
represent the query cache and net->query_cache_query can not be used
any more (due to ABI compatibility issues and to different life
time of NET and THD).
This is a backport of the following patch from 6.0:
----------------------------------------------------------
revno: 2476.1157.2
committer: kostja@bodhi.(none)
timestamp: Sat 2007-06-16 13:29:24 +0400


include/mysql.h.pp:
  Update the ABI (NET::query_cache_query is now unused).
include/mysql_com.h:
  NET::query_cache_query is now unused.
sql/mysql_priv.h:
  Update signatures of ex-functios now member functions.
sql/sql_cache.cc:
  Introduce THD::query_cache_tls.
sql/sql_cache.h:
  Introduce THD::query_cache_tls.
sql/sql_class.cc:
  Introduce THD::query_cache_tls.
sql/sql_class.h:
  Introduce THD::query_cache_tls.
sql/sql_parse.cc:
  Use THD::query_cache_tls.
sql/sql_select.cc:
  Use THD::query_cache_tls.