When @@session.binlog_format is modified inside a transaction,
it can cause slave to go out of sync.
      
To fix the problem, make the session variable 'binlog_format' 
read-only inside a transaction.


mysql-test/suite/binlog/r/binlog_format_switch_inside_trans.result:
  Test result for bug#47863.
mysql-test/suite/binlog/t/binlog_format_switch_inside_trans.test:
  Added test file to verify if the session variable 'binlog_format' 
  is read-only inside a transaction and in sub-statements.
sql/set_var.cc:
  Added code to make the session variable 'binlog_format'
  read-only inside a transaction.