The thd->variables.option_bits & OPTION_BIN_LOG is currently abused: 
it's both a system variable and an implementation switch. The current
approach to this option bit breaks the session variable encapsulation. 
      
Besides it is allowed to change @@session.sql_bin_log within a
transaction what may lead to not correctly logging a transaction.
      
To fix the problems,  we created a thd->variables variable to represent
the "sql_log_bin" and prohibited its update inside a transaction or
sub-statement.

mysql-test/suite/binlog/r/binlog_stm_unsafe_warning.result:
  Updated result file. The reason the warnings are removed is
  related to BUG#50312.
mysql-test/suite/binlog/r/binlog_switch_inside_trans.result:
  Checks when is possible to change the option @@session.sql_log_bin.
mysql-test/suite/binlog/t/binlog_switch_inside_trans.test:
  Checks when is possible to change the option @@session.sql_log_bin.
mysql-test/suite/rpl/r/rpl_non_direct_stm_mixing_engines.result:
  Updated the result file with warnings that were being printed
  due to the wrong use of the thd->variables.option_bits and
  sql_log_bin_top_level variables.
mysql-test/suite/rpl/r/rpl_stm_mixing_engines.result:
  Updated the result file with warnings that were being printed
  due to the wrong use of the thd->variables.option_bits and
  sql_log_bin_top_level variables.
sql/share/errmsg-utf8.txt:
  Introduces two error messages to notify that the @@session.sql_log_bin
  cannot be changed inside a sub-statement or transaction.
sql/sql_base.cc:
  Removes the variable sql_log_bin_toplevel and uses
  the session variable sql_log_bin.
sql/sql_class.cc:
  Replaces the variable sql_log_bin_toplevel by the
  (variables.option_bits & OPTION_BIN_LOG).
sql/sql_class.h:
  Removes the variable sql_log_bin_toplevel and creates 
  a session variable sql_log_bin.
sql/sys_vars.cc:
  Checks when the sql_log_bin can be correctly updated.