sql/sp.cc:
  In-memory cache added.
  sp_clear_function_cache() no longer needed.
sql/sp.h:
  In-memory cache added.
  sp_clear_function_cache() no longer needed.
sql/sql_class.cc:
  In-memory cache added.
sql/sql_class.h:
  In-memory cache added.
sql/sql_parse.cc:
  In-memory cache added.
  sp_clear_function_cache() no longer needed.
  Don't delete sp_heads after each use.