Freeze the number of conflicts. We should not introduce any more

conflicts without proper analysis and documentation of the nature of
the conflict.

sql/sql_yacc.yy

@@ -389,6 +389,11 @@ bool my_yyoverflow(short **a, YYSTYPE **b, ulong *yystacksize);
 %}
 
 %pure_parser					/* We have threads */
+/*
+  Currently there is 251 shift/reduce conflict. We should not introduce
+  new conflicts any more.
+*/
+%expect 251
 
 %token  END_OF_INPUT