Engine-independent indexes: low-level API method

This tasks covers an alternative to MDEV-33404:

Defining a low-level interface for storage engines to allow for page allocation

The purpose of this interface is to support lower-level implementation of indexing strategies, should the subtable (high level interface) prove too slow.

High level overview
The index will ask the engine for a page in the table space and can store the index there in any arbitrary way it wants, the engine will treat it as opaque.

Benefits

• Faster and more flexible than MDEV-33404

Drawbacks

• The burden of ACID is on the index implementation, it handles undo/redo, recovery, isolation
  • This has to be done for every supported engine separately
• Works only for very few selected engines, may be only InnoDB
  • So not very engine independent

Interface questions to be answered:
What operations should this interface support?

1. Alloc page(s) (implemented by SE)
2. Free page(s) (implemented by SE)
3. Register callback for row insert/update/delete (callback invoked by SE when event occurs)
4. Register callback for transaction start/commit/rollback (callback invoked by SE when event occurs)

Does Engine query the Index or is it only for the upper SQL layer level to decide?