Added following:

        FLUSH CLIENT_STATISTICS
        FLUSH INDEX_STATISTICS
        FLUSH TABLE_STATISTICS
        FLUSH THREAD_STATISTICS
        FLUSH USER_STATISTICS

pertaining to USER STATISTICS
https://www.percona.com/doc/percona-server/5.6/diagnostics/user_stats.html

        FLUSH CHANGED_PAGE_BITMAPS

pertaining to changed page tracking.
https://www.percona.com/doc/percona-server/5.6/management/changed_page_tracking.html

Also, added tests for them.

(cherry picked from commit 7efe49010c7f217663f364657090812b4723f426)

Conflicts:
	mysql-test/suite/galera/r/galera_flush.result
	mysql-test/suite/galera/r/galera_flush_gtid.result
	mysql-test/suite/galera/t/galera_flush.test
	sql/sql_parse.cc