Add support for on-the-fly document upgrades

[JKamsker]

This change would allow me to improve my library (https://github.com/JKamsker/LiteDb.Migration) significantly. The documents would then only be upgrades on load and not all at once.

[JKamsker]

This also addresses #2430

[JKamsker]

@pictos Thanks for the review, how did that slip through, that its a breaking change 😅 Reverted the breaking changes and updated your requested changes.

So in my latest commit i added a deserialization hook.

Upsides:

• It is easier to add your own hook into that, because you only need a reference to the mapper
• Should be easier on the cpu because it only transforms those documents, which are actually needed.

Downsides:

• Queries may not run as expected, because there could be a transform between the query engine and the actual transform. But if that is a concern, the user should just use the ReadTransform in the engine settings.

[JKamsker]

Some tests failed before which are succeding (not the tests i touched)...sooo...

@pictos Can we merge? 😅