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

[pictos]

Looking into the issue for more context, I think is better to @mbdavid to take a look on this and see if he wants to perform this change, or if using the protected methods are enough

[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.

[pictos]

I'm away from my pc, so I can't test it right now

[JKamsker]

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

@pictos Can we merge? 😅

[mbdavid]

To keep consicise with all code base, let's use "this." for instance methods, properties and public fields (only private _ name has no "this.")