Also fixed some difficult memory leaks that became apparent
in this task.


sql/sp.cc:
  sp_head now has its own mem_root.
sql/sp_head.cc:
  sp_head now has its own mem_root.
  Also fixed some difficult memory leaks.
sql/sp_head.h:
  sp_head now has its own mem_root.
sql/sql_lex.h:
  Fixed some memory leaks in sp_head. Need to keep track on used lex:es.
sql/sql_parse.cc:
  sp_head now has its own mem_root.
  Fixed SP memory leaks.
sql/sql_prepare.cc:
  Fixed SP memory leaks.
sql/sql_yacc.yy:
  sp_head now has its own mem_root.