Implement MarkM optimization request to avoid redundnat packet exchange

in cursors.


libmysql/libmysql.c:
  - reset_stmt_handle(): don't reset the server side just because we have 
    an open cursor: the server will close the cursor automatically if 
    needed
sql/sql_prepare.cc:
  - implement Prepared_statement::close_cursor,
  - implicitly close an open cursor in mysql_stmt_execute instead of 
    issuing an error (to reduce the need to explicitly close cursors
    and save network bandwidth).
  - cleanup
sql/sql_select.cc:
  Remove a destructor: cursor destruction can not be done by simply
  calling a destructor, because of cross-references between cursor
  and statement memory.
sql/sql_select.h:
  - add an empty Cursor destructor
tests/mysql_client_test.c:
  - remove a test for dropped functionality

sql/sql_select.h

@@ -408,7 +408,7 @@ public:
 
   void set_unit(SELECT_LEX_UNIT *unit_arg) { unit= unit_arg; }
   Cursor(THD *thd);
-  ~Cursor();
+  ~Cursor() {}
 };