BUG#46364 MyISAM transbuffer problems (NTM problem)

It is well-known that due to concurrency issues, a slave can become
inconsistent when a transaction contains updates to both transaction and
non-transactional tables.
                    
In a nutshell, the current code-base tries to preserve causality among the
statements by writing non-transactional statements to the txn-cache which
is flushed upon commit. However, modifications done to non-transactional
tables on behalf of a transaction become immediately visible to other
connections but may not immediately get into the binary log and therefore
consistency may be broken.
            
In general, it is impossible to automatically detect causality/dependency
among statements by just analyzing the statements sent to the server. This
happen because dependency may be hidden in the application code and it is
necessary to know a priori all the statements processed in the context of
a transaction such as in a procedure. Moreover, even for the few cases that
we could automatically address in the server, the computation effort
required could make the approach infeasible.
            
So, in this patch we introduce the option
      - "--binlog-direct-non-transactional-updates" that can be used to bypass
      the current behavior in order to write directly to binary log statements
      that change non-transactional tables.

Besides, it is used to enable the WL#2687 which is disabled by default.

sql/sql_class.h

@@ -373,6 +373,7 @@ typedef struct system_variables
   ulong group_concat_max_len;
 
   uint binlog_format; ///< binlog format for this thd (see enum_binlog_format)
+  my_bool binlog_direct_non_trans_update;
   uint completion_type;
   uint query_cache_type;
   uint tx_isolation;