Extend index-only prefix index query optimisation to secondary indexes on generated columns

[Laurynas Biveinis]

Percona Server 5.7 has ported Facebook prefix index query optimisation (https://github.com/percona/percona-server/pull/1505, https://github.com/percona/percona-server/pull/1506).

Extend this optimisation to apply to secondary indexes on generated columns too. A superficial review seems to show that this might be involved, as server always requests base column image from the storage engine on the queries that this optimisation might apply.

[Laurynas Biveinis (laurynas-biveinis)]

Relevant 5.7 worklogs:

    WL#8149 B-tree Index Support on non-materialized virtual columns
    WL#8114 Don't store virtual generated columns in database
    WL#8227 Support SEs to create index on virtual generated columns
    WL#8481 Callback for computation of virtual column index values
    from InnoDB purge threads
    All 4 worklogs are to support virtual column and virtual index on such
    columns.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-2497