WL#7743: New data dictionary: changes to DDL-related parts of SE API

Affects: Server-8.0   —   Status: Complete

As part of the New Data-Dictionary project, in order to allow InnoDB to get rid
of its internal data-dictionary and support atomic/crash-safe DDL, we need to
extend SQL-layer code and parts of SE API which are related to opening tables
and DDL.

The following needs to be supported:

1) Auxiliary columns and keys (hidden system columns and keys
   which InnoDB adds to the tables implicitly).
2) Access to se_private_* values for DD objects during opening
   tables and updating them during DDL.
3) Atomic/crash-safe DDL. While implementing this item it also makes
   sense to change DDL user-visible behavior to more atomic.

InnoDB also needs to be able to store info about auxiliary tables (needed
for FTS) in the DD. However it seems that existing support for this in DD
API is sufficient for now.