There was memory leak when running some tests on PB2.
The reason of the failure is an early return from change_master()
that was supposed to deallocate a dyn-array.

Fixed with relocating the dyn-array's destructor at ~LEX() that is
the end of the session, per Gleb's patch idea.
Two optimizations were done: the static buffer for the dyn-array to base on,
and the array initialization is called precisely when it's necessary rather than
per each CHANGE-MASTER as before.
 

mysql-test/suite/rpl/t/rpl_empty_master_host.test:
  the test is binlog-format insensitive so it will be run with MIXED mode only.
sql/sql_lex.cc:
  the new flag is initialized.
sql/sql_lex.h:
  A new bool flag new member to LEX.mi is added to stay UP since after
  LEX.mi.repl_ignore_server_ids dynarray initialization was called
  for the first time on the session. So it is set once and its life time 
  is session.
  
  The array is destroyed at the end of the session.
sql/sql_repl.cc:
  dyn-array destruction is relocated to ~LEX.
sql/sql_yacc.yy:
  Refining logics of Lex->mi.repl_ignore_server_ids initialization.
  The array is initialized once a corresponding option in CHANGE MASTER token sequence
  is found.
  The fact of initialization is memorized into the new flag.