* Moved data locking in the internal buffer class, includes handling
of backend writes.
* Takes advantage of inheritance for getting the size. The choice is
made depending on the flags. This allow the implementer to easily
return a custom size by reimplementing GetSize. At the same time
a plain BAdapterIO can still have it's total size set, but the behavior
will change depending it's mutable or not.
* Some decisions are now made by considering everything in absolute
values.
* Other minor fixes.
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