WL#3960: Online backup: Engine-agnostic image format

Affects: Server-6.0   —   Status: On-Hold   —   Priority: Medium

Make it possible to restore a CS/default backup of one engine on
another engine.

- Default driver must save data independent of storage engine
- Consistent snapshot driver must store data independent of storage engine

Currently the default and CS backup drivers store rows read
from a table in the raw format used by the storage engine. 

We should use more engine-agnostic format which is used now in
row-based replication. Mats has written methods for transforming between two
formats and we can easily utilize them in the drivers. 

However, we must think about future backward compatibility, 
in case the record storage format changes due to development 
in the replication code. Our tool for that is image 
version numbers.

WL#4749 has been created for testing this property.