Moved PS name to Statement class, Statement_map now handles name-to-statement resolution.
  Both named and unnamed statements are now executed in one function (sql_prepare.cc:execute_stmt)
  Fixed a problem: Malformed sequence of commands from client could cause server to use previously deleted objects.
  Some code cleanup and small fixes


sql/mysql_priv.h:
  WL#1622 "SQL Syntax for Prepared Statements": post-review fixes.
sql/sql_class.cc:
  WL#1622 "SQL Syntax for Prepared Statements": post-review fixes.
sql/sql_class.h:
  WL#1622 "SQL Syntax for Prepared Statements": post-review fixes.
sql/sql_parse.cc:
  WL#1622 "SQL Syntax for Prepared Statements": post-review fixes.
sql/sql_prepare.cc:
  WL#1622 "SQL Syntax for Prepared Statements": post-review fixes.