WL#8321: Enable statements_history and transactions_history consumers by default in 5.7

This work changes the defaults of 
- performance-schema-consumer-events-statements-history to "ON" (was: OFF) 
- performance-schema-consumer-events-transactions-history to "ON" (was: OFF)

This produces information that might be needed to debug a variety of query
related issues. 

User Documentation
==================

http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-7.html
http://dev.mysql.com/doc/refman/5.7/en/performance-schema-filtering.html
http://dev.mysql.com/doc/refman/5.7/en/performance-schema-statement-tables.html
http://dev.mysql.com/doc/refman/5.7/en/performance-schema-transaction-
tables.html

Change the default value to ON for the following configuration parameters:

performance-schema-consumer-events-statements-history=ON
performance-schema-consumer-events-transactions-history=ON

Change the default value to ON for the following configuration parameters:

performance-schema-consumer-events-statements-history=ON
performance-schema-consumer-events-transactions-history=ON