The BINLOG statement was sharing too much code with the slave SQL thread, introduced with
the patch for Bug#32407. This caused statements to be logged with the wrong server_id, the
id stored inside the events of the BINLOG statement rather than the id of the running 
server.
      
Fix by rearranging code a bit so that only relevant parts of the code are executed by
the BINLOG statement, and the server_id of the server executing the statements will 
not be overrided by the server_id stored in the 'format description BINLOG statement'.

mysql-test/extra/binlog_tests/binlog.test:
  Added test to verify if the server_id stored in the 'format 
  description BINLOG statement' will override the server_id
  of the server executing the statements.
mysql-test/suite/binlog/r/binlog_row_binlog.result:
  Test result for bug#46640
mysql-test/suite/binlog/r/binlog_stm_binlog.result:
  Test result for bug#46640
sql/log_event.cc:
  Moved rows_event_stmt_clean() call from update_pos() to apply_event(). This in any case
  makes more sense, and is needed as update_pos() is no longer called when executing
  BINLOG statements.
  
  Moved setting of rli->relay_log.description_event_for_exec from 
  Format_description_log_event::do_update_pos() to 
  Format_description_log_event::do_apply_event()
sql/log_event_old.cc:
  Moved rows_event_stmt_clean() call from update_pos() to apply_event(). This in any case
  makes more sense, and is needed as update_pos() is no longer called when executing
  BINLOG statements.
sql/slave.cc:
  The skip flag is no longer needed, as the code path for BINLOG statement has been 
  cleaned up.
sql/sql_binlog.cc:
  Don't invoke the update_pos() code path for the BINLOG statement, as it contains code 
  that is redundant and/or harmful (especially setting thd->server_id).