InnoDB page fragmentation counters (5.7)

https://github.com/percona/percona-server/pull/1953

Port' innodb_check_fragmentation.patch' patch from Percona Server 5.0 to 5.7
https://www.percona.com/blog/2009/11/05/innodb-look-after-fragmentation/

Reimplement the following InnoDB status variables as in the
https://gitweb.gentoo.org/proj/mysql-extras.git/tree/percona/5.0.91-b22-20100522/innodb_check_fragmentation.patch

* 'innodb_scan_pages_contiguous' - number of contiguous page reads inside a query.
* 'innodb_scan_pages_disjointed' - number of disjointed page reads inside a query.
* 'innodb_scan_data_size' - size of data in all InnoDB pages read inside a query (in bytes) - calculated as the sum of 'page_get_data_size(page)' for every page scanned.
* 'innodb_scan_deleted_recs_size' - size of deleted records (marked as 'deleted' in 'page_delete_rec_list_end()') in all InnoDB pages read inside a query (in bytes) - calculated as the sum of 'page_header_get_field(page, PAGE_GARBAGE)' for every page scanned.

Introduce new
* 'innodb_scan_pages_total_seek_distance' - total seek distance when moving between pages.

These variables are accumulated on per-session level and are updated after each query.